Shorelines and Vertebrate Fauna of Pleistocene Lake Bonneville, Utah, Idaho, and Nevada GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 4 2017 © 2017 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. SHORELINES AND VERTEBRATE FAUNA OF PLEISTOCENE LAKE BONNEVILLE, UTAH, IDAHO, AND NEVADA Mark Milligan and H. Gregory McDonald A Field Guide Prepared For SOCIETY OF VERTEBRATE PALEONTOLOGY Annual Meeting, October 26 – 29, 2016 Grand America Hotel Salt Lake City, Utah, USA GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover Restored outline of Lake Bonneville. Geological data by G.K. Gilbert and E.E. Howell. Lithography by J. Bien. Dated 1876. From, “Topographical Atlas Projected to Illustrate United States Geographical Surveys West of �e 100th Meridian,” a collection of 135 topographi- cal and geological atlas sheets, 1876 to 1881. Down- loaded from the David Rumsey Map Collection, www. davidrumsey.com. Ancient lake surface in light blue. Present (1876) lake surfaces and running water cours- es in dark blue. Ancient land surface in dark drab. �ought to be the oldest published map of Lake Bon- neville, this 1876 map does not show the full extent of the lake as depicted in Gilbert’s 1890 monograph. i 2017 President Bill Loughlin bill@loughlinwater.com 435.649.4005 2017 President-Elect Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 2017 Program Chair Andrew Rupke andrewrupke@utah.gov 801.537.3366 2017 Treasurer Robert Ressetar rrgeology@gmail.com 801.949.3312 2017 Secretary Tom Nicolaysen tnicolaysen@utah.gov 801.538.5360 2017 Past-President Jason Blake blake-j@comcast.net 435.658.3423 UGA Board UGA Committees Education/Scholarship Loren Morton lmorton@utah.gov 801.536.4262 Environmental A�airs Craig Eaton eaton@ihi-env.com 801.633.9396 Geologic Road Sign Terry Massoth twmassoth@hotmail.com 801.541.6258 Historian Paul Anderson paul@pbageo.com 801.364.6613 Membership Rick Ford rford@weber.edu 801.626.6942 Public Education Paul Jewell pwjewell@mines.utah.edu 801.581.6636 Matt A�olter g�247@yahoo.com Publications Roger Bon rogerbon@xmission.com 801.942.0533 Publicity Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Social/Recreation Roger Bon rogerbon@xmission.com 801.942.0533 AAPG House of Delegates 2017-2020 Term Tom Chidsey tomchidsey@utah.gov 801.537.3364 State Mapping Advisory Committe UGA Representative Jason Blake blake-j@comcast.net 435.658.3423 UGA Newsletter Newsletter Editor Bob Biek bobbiek@utah.gov 801.537.3356 UGA Website www.utahgeology.org Webmasters Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Lance Weaver lanceweaver@utah.gov 801.403.1636 Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual �eld conferences, and new publi- cations. Annual membership is $20 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. �e UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. If you would like to participate please contact the current president or committee member corresponding with the area in which you would like to volunteer. Utah Geological Association formed in 1970 from a merger of the Utah Geological Society, founded in 1946, and the Intermountain Association of Geologists, founded in 1949. A�liated with the American Association of Petroleum Geologists. Volume 4 2017 �is is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and �gures that are not noted as copyrighted, provided the original author and source are credited. Earthquake Safety Committe Chair Grant Willis gwillis@utah.gov 801.537.3355 Douglas A. Sprinkel Utah Geological Survey 801.391.1977 GIW@utahgeology.org Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com �omas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov Steven Schamel GeoX Consulting, Inc. 801.583-1146 geox-slc@comcast.net Society of Vertebrate Paleontology Editors James I. Kirkland (Editor-in-Chief) — Utah Geological Survey ReBecca Hunt-Foster — Bureau of Land Management Greg McDonald — Bureau of Land Management Martha Hayden — Utah Geological Survey Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 4 2017 181 ABSTRACT Pleistocene Lake Bonneville created many classic examples of lacustrine shoreline landforms, which preserve a wide variety of vertebrate fossils. �is �eld guide provides a review of the published literature for a sampling of the lake’s world-class localities. �is guide also provides a brief overview of modern Great Salt Lake and its microbialites recently exposed by near-record low lake levels. Stops include G.K. Gilbert Geologic View Park, Draper spit, Steep Mountain beach, Point of the Mountain spit, American Fork delta, Stockton Bar, and Great Salt Lake State Park. Shorelines and Vertebrate Fauna of Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Mark Milligan1 and H. Gregory McDonald2 1Utah Geological Survey, P.O. Box 146100, Salt Lake City, UT 84116; markmilligan@utah.gov 2Bureau of Land Management, Utah State O�ce, 440 W. 200 S., Salt Lake City, UT 84101; hmcdonald@blm.gov Citation for this article. Milligan, M., and McDonald, H.G., 2017, Shorelines and vertebrate fauna of Pleistocene Lake Bonneville, Utah, Idaho, and Nevada: Geology of the Intermountain West, v. 4, p. 181–214. © 2017 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org INTRODUCTION TO LAKE BONNE- VILLE AND THE BONNEVILLE BASIN �e following section is modi�ed from Godsey and others, 2005a. Lake Bonneville was a large pluvial lake that occu- pied the eastern Great Basin during latest Pleistocene marine Oxygen Isotope Stage (MIS) 2, from approxi- mately 30 to 13 cal ka (�gure 1) (Gilbert, 1890; Ovi- att, 2015). �is region is part of the Basin and Range Province that developed in response to east-west crust- al extension beginning ca. 20 Ma. Normal faulting and subsidence caused the formation of basins separated by upli�ed fault blocks. �e most signi�cant subsid- ence occurred at the eastern margin of the Basin and Range Province, and it is here that the Bonneville basin formed. Extension continues to modify the landscape in the Basin and Range today. �e Bonneville basin has been a region of internal drainage for the past 15 m.y. and lakes of varying sizes have occupied part of the basin throughout most of this Figure 1. Simpli�ed hydrograph of Lake Bonneville. Alti- tudes adjusted for isostatic rebound. Bars at top show trans- gressive (T), over�owing (O), and regressive (R) phases. Modern Great Salt Lake (GSL) average altitude is 1280 m. Lake Gunnison is an isolated lake in the Sevier sub-basin that formed with the regression of Lake Bonneville. Figure courtesy of Jack Oviatt (Kansas State Univeristy). www.utahgeology.org 182 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 time (Currey and others, 1984). Researchers have pro- duced evidence that at least four deep-lake cycles have occurred in the Bonneville basin over the past 780,000 years, the Bonneville lake cycle being the youngest of these (Oviatt and others, 1999). Lake Bonneville began to form ca. 30 ka when colder and/or wetter climate conditions during the Last Glacial Maximum caused it to rise (Oviatt, 2015). �e timing and/or the rapidity of the rise may have been in�uenced by diversion of the upper Bear River into the Bonneville basin ca. 30 ka (Reheis and others, 2014), although Ped- erson and others (2016) report evidence that the river diversion occurred about 50 ka. �e transgression con- tinued until ca. 25 ka when changing climate conditions caused the lake to undergo a slight regression (Oviatt, 2015). For nearly 3000 yr, the level of the lake oscillat- ed through a vertical range of about 50 m near 1370 m above sea level (masl), forming the Stansbury shoreline, and then began to rise again. By ca. 18 ka, the lake had reached the level of the topographic divide near Zenda, Idaho (�gure 2), and began to over�ow. �is marks the beginning of the open-basin phase of the lake and the formation of the Bonneville shoreline (�gure 1). When the lake became very deep and heavy in the center of the basin, transgression was enhanced by hy- dro-isostatic subsidence. However, the primary driver of transgression was a change in water budget caused by climate change in the basin. Water inputs from precip- itation, groundwater, and runo� became much greater than outputs by evaporation and groundwater. At 18 ka Lake Bonneville was over 300 m deep and covered an area of ~51,000 km2 (Oviatt and Miller, 1997). Rivers emanating from the high mountains to the east provid- ed large amounts of clastic material that was deposited in deltas along the mountain fronts (Lemons and oth- ers, 1996). Spits, barriers, bars, and beaches were sup- plied sediment by alluvium and weathered bedrock in regions with little riverine input. Muds and marls were deposited in more distal, o�shore locations (Oviatt and Miller, 1997). Catastrophic failure of the alluvial-fan dam (the al- luvial-fan deposits overlie tu�aceous sediments of the Neogene Salt Lake Formation) at the Zenda (�gure 2) threshold in southern Idaho ca. 18 ka caused a massive �ooding of lake waters into the Snake River drainage (Gilbert, 1890). �ere is insu�cient resolution to deter- mine how long the lake remained at the threshold be- fore failure, although it is likely the over�ow period was Figure 2. Shaded-relief map showing the ex- tent of Lake Bonneville and other Great Ba- sin lakes (blue) and select mountain ranges with valley glaciers (white) during the latest Pleistocene. Dashed line delineates Great Salt Lake. Red boxes indicate the locations, Little Cottonwood (LC) and American Fork (AF) Canyons, of study areas for 10Be cosmogenic surface-exposure dating of erratic boulders atop moraines by Laabs and others (2011) and Laabs and Munroe (2016). Modi�ed from Laabs and others (2011). 183 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 very short (Oviatt, 2015; Oviatt and Jewell, 2016). �is event, called the Bonneville �ood, caused a drop in the lake level of at least 125 m (Miller and others, 2013) and is believed to have occurred in less than one year’s time (Malde, 1968; O’Connor, 1993, 2016). Headward ero- sion by �ood waters shi�ed the drainage divide to the southeast ~3.2 km to near Red Rock Pass, Idaho (Cur- rey, 1982). �e lake re-stabilized at the new threshold, probably on massive landslide deposits, and the Provo shoreline began to form (�gure 1; Gilbert, 1890; Miller, 2016). �e lake oscillated at or near the level of the Red Rock Pass outlet until a change in climate conditions permanently drove the lake below the threshold, start- ing ca. 15 ka (Godsey and others, 2011; Oviatt, 2015). By ca. 13 ka, the lake had reached levels comparable to modern Great Salt Lake (Oviatt, 2015). �is marks the end of the Bonneville lake cycle and the beginning of its successor, Great Salt Lake. �e lake appears to have risen again brie�y ca. 11.6 ka during the Gilbert epi- sode (Oviatt and others, 2005; Oviatt, 2014), but the ex- istence and extent of a Gilbert shoreline is suspect and the subject of ongoing investigations (Oviatt, 2014). �e mass of Lake Bonneville’s water weighed down and isostatically depressed the Earth’s crust. With the demise of the lake the crust rebounded to its original elevation, thus elevating and distorting the originally horizontal shorelines (Gilbert, 1890; Adams and Bills, 2016). �e greatest rebound, up to 74 m, was at the deepest part of the basin on the west side of Great Salt Lake (Currey, 1990). CLIMATE AND GLACIATION �ough permanent diversion of the upper Bear Riv- er into the Bonneville basin may have in�uenced the timing and/or the rapidity of the rise of Lake Bonneville (Reheis and others, 2014), the climatic variables of evap- oration and precipitation subsequently controlled lake level and glacial extent in the Bonneville basin (Laabs and Munroe, 2016). �e importance of temperature vs. precipitation in controlling lake and glacier mass bal- ance has been controversial (Laabs and others, 2011; Reheis and others, 2014). Some studies suggest a cold and dry climate in the interior western U.S. during MIS 2 (e.g., Porter and others, 1983; Kaufman, 2003). Other studies suggest increased precipitation drove lake and glacier expansions (e.g., Benson and �ompson, 1987). �e long-held idea that glacial maxima predated lake-level highstand (e.g., Madsen and Currey, 1979) favored a wetter and warmer scenario. However, God- sey and others (2005b) describe glacial till stratigraph- ically above and below Bonneville shoreline deposits. Furthermore, Laabs and Munroe (2016) used 10Be cos- mogenic surface-exposure dating of erratic boulders to determine that terminal moraines in the Bonneville ba- sin were occupied near the time of the Bonneville high- stand at 18 ka and subsequently abandoned while the lake continued to over�ow at the Provo threshold. �us, the climate favored simultaneous lake trans- gression and glacial advance until ca. 18 ka. �e ap- parent start of ice retreat while the lake still over�owed suggests that sometime a�er 18 ka the climate warmed but precipitation remained high enough to sustain a positive water budget for the lake until retreat from the Provo shoreline at ca. 15 ka (Laabs and Munroe, 2016). THE PLEISTOCENE AND HOLOCENE VERTEBRATE FAUNA OF LAKE BONNEVILLE In Gilbert's 1890 monograph on the geology of Lake Bonneville, it only brie�y mentions vertebrate remains as some “elephantine bones and ivory” recovered from a “post-Bonneville” marsh east of Utah Lake. However, Pleistocene vertebrate remains associated with the lake deposits were being discovered in the area during the earliest studies of the lake (Chadbourne, 1871) and the recovery of vertebrate fossils from Lake Bonneville sed- iments and sediments in caves in the Bonneville basin continues to the present (�gure 3). Our knowledge of the Pleistocene vertebrate fauna associated with Lake Bonneville is based on numerous localities in the lake basin. Je�erson and others (1994) listed 18 localities in Salt Lake County, three in Davis County, and one each in Box Elder and Weber Coun- ties. Most of our knowledge of the fauna has occurred recently (Stokes and others, 1964; Smith and others, 1968; Nelson and Madsen, 1978, 1980, 1983, 1986, 1987; Miller, 1982, 2002; Feduccia and Oviatt, 1986; Gillette and Miller, 1999; Schmitt and Lupo, 2016). Based on the 184 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 location, elevations, and the physical stratigraphy of the various sites, the majority of the fauna associated with Lake Bonneville gravel deposits comes from sediments deposited at or near the Bonneville and Provo shore- lines (Nelson and Madsen, 1987; McDonald and others, 2001). A�er dropping from the Bonneville to Provo level, the lake occupied the Provo shoreline for about 3000 years. �e Provo shoreline is formed by a complex of several coalescing coastal landforms, many of which form a pattern of distinctive beach ridges that can be identi�ed throughout the basin (Burr and Currey, 1988; Godsey and others, 2005a; Miller and others, 2013; Miller, 2016). Radiocarbon ages of sediments associat- ed with the Provo shoreline indicate that Lake Bonne- ville dropped rapidly from the Provo shoreline at about 12,600 14C yr B.P. (15,000 cal yr B.P.). �e rapid lowering of the lake level from the Provo shoreline correlates with the decline of Lakes Lahontan in Nevada and Estancia in New Mexico, and with the onset of the Bølling–Allerød warming event (Godsey and others, 2011). �e earliest report of Pleistocene vertebrates from deposits associat- ed with Lake Bonneville was by King (1878) who noted that “the latest subaerial gravels [i.e., Provo shoreline] yielded a skull of Bison latifrons and fragments of bones, supposed to be a “reindeer.” Unfortunately the location where the specimens were found is unknown as is the current location, and it is not known if they were even collected. Bison antiquus is also present and has been recovered from gravel pits along the eastern shoreline of the lake (�gure 4B). �e deposits associated with the Provo shoreline span the time interval when the climate in the Great Ba- sin was transitioning from the colder, wetter conditions of the Last Glacial Maximum to the warmer, drier con- ditions of the late Pleistocene and early Holocene (God- sey and others, 2011). �e expansion and contractions of lake levels in the Great Basin during the late Pleisto- cene to Holocene transition have been linked to position of the jet stream as it was de�ected around the North American ice sheets (Kutzbach, 1987). In general, plu- vial lakes at lower latitudes (ca. 32°–35° N), such as Lake Estancia in New Mexico, reached their highstand when the continental ice sheets had reached their maximum extent about 15,000 14C years ago (Garcia and Stokes, 2006). In contrast, lakes at higher latitudes (ca. 38°–40° N), such as Bonneville and Lahontan, experienced their Figure 3. �e Bonneville basin showing locations of select- ed paleontological and archaeological sites with vertebrate remains. Circles are open air sites and triangles are cave or rock-shelter sites: (1) Franklin Peccary Site, (2) Logan Cemetery, (3) Hogup Cave, (4) Homestead Cave, (5) Danger Cave, (6) Bonneville Estates Rock Shelter, (7) Camels Back Cave, (8) Lark, (9) Jordan Narrows, (10) Point of the Moun- tain Sloth Site, (11) Monroc Gravel Pit, (12) Huntsman, (13) Sandy Mammoth, (14) Orem Sloth Site, (15) Spanish Fork, (16) Tabernacle Crater Camel Site, (17) Crystal Ball Cave, (18) Smith Creek Cave and Cathedral Cave, and (19) Snake Creek Burial Cave. Modi�ed from Schmitt and Lupo (2016). 185 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 Figure 4. Pleistocene vertebrate fossils from gravel pits on the east side of Lake Bonneville. (A) Camelops hesternus, prox- imal phalanx, anterior view. (B) Bison antiquus metacarpal, anterior view. (C) Arctodus simus right tibia, anterior view. (D and E) Equus sp. braincase, right lateral, posterior view. (F and G) Ovis canadensis braincase, anterior and right lateral view. (H and I) Mammuthus columbi right upper molar, occlusal, lateral view. (J and K) Bootherium bombifrons (male) braincase, posterior and dorsal view. All specimens in the Natural History Museum of Utah. Scale is 5 cm. 186 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 highstands slightly later (ca. 14,500–13,500 14C yr B.P.) following the retreat of the ice sheets northward (Ad- ams and Wesnousky, 1998; Benson and others, 1990; Garcia and Stokes, 2006). As the ice sheets continued to retreat, storm tracks also shi�ed northward, resulting in reduced annual precipitation and a lowering of water levels in pluvial lakes in the region. �e development of a clear record of changes in the lake-level within the Bonneville basin is critical for examining regional climate patterns and the degree of synchronicity in the highstands in the numerous pluvial lakes across the western United States during the Last Glacial Maximum (Benson and others, 1990; Adams and Wesnousky, 1998; Benson, 1999; Licciardi, 2001; Garcia and Stokes, 2006; Broecker and others, 2009). A better understanding of the climatic conditions that determined lake levels also provides a framework for understanding how the Pleistocene fauna of the region was impacted by climatic and environmental change since changes in precipitation not only impacted lake levels but vegetation as well. Multiple caves (e.g., Homestead Cave and Cathedral Cave in Utah, and Danger Cave and Smith Creek Cave in Nevada) are associated with Lake Bonneville that af- ter becoming exposed with the lowering of lake level accumulated small vertebrate remains. Sediments in Homestead Cave have been dated from 11,270 ± 135 to 1020 ± 40 14C yr B.P. (Broughton, 2000). Radiocarbon dates for Cathedral Cave range from 15,310 ± 60 to 740 ± 40 14C yr B.P. (Madsen, 2000). Whereas the fauna re- covered from both caves spanned the Pleistocene-Holo- cene transition no extinct species were recovered from either cave. �e composite fauna which is derived from multi- ple sites spanning thousands of years is highly biased toward larger taxa; 15 mammals, two birds, and six �sh are reported by Nelson and Madsen (1987). Since then the megafauna has been expanded by the discovery of two separate �nds of the Je�erson ground sloth, Megalo- nyx je�ersonii (Gillette and others, 1999; McDonald and others, 2001). Whereas some specimens can be placed within a chronology based on where they were collect- ed, the vertebrae faunal record is not su�ciently robust or has a tight enough chronology at this time to actual- ly document any faunal change in response to climatic change. Few of the older specimens have been radiocar- bon dated, and in many cases the quality of preserva- tion of many specimens does not permit a radiocarbon date. Schmitt and Lupo (2016, table 13.1) provide a list of the extinct Pleistocene mammals of the Bonneville basin, although some of the taxa were recovered from localities adjacent to the basin and not the basin proper. A composite list of taxa from sites directly associated with Lake Bonneville appears in table 1. A muskox skull was among the �rst vertebrate fos- sils to be described from the Bonneville basin (Chad- bourne, 1871). Nelson and Madsen (1987) recognized two taxa of musk ox, Bootherium bombifrons and Sym- bos cavifrons, from the Bonneville basin but these two taxa are now considered male (Symbos) and female (Bootherium) of the same species (McDonald and Ray, 1989), so one extinct species of muskox, Bootherium bombifrons, is now recognized from the late Pleisto- cene in North America, along with the extant muskox, Ovibos moshatus. Nelson and Madsen (1987) reported four skulls of the female have been recovered and 21 skulls of the male along with some post-cranial materi- al. Gillette and Miller (1999) reported a partial skeleton found during construction of the Huntsman Chemical Corporation headquarters in Salt Lake City, another skull from the Staker gravel pit, an ear region and ver- tebrae from a gravel pit owned by the Kennecott Cop- per Company in Salt Lake County, a skull from Spanish Fork in Utah County, and post-cranial bones from the same gravel pit at Point of the Mountain that produced a ground sloth. �e predominance of male skulls has been noted elsewhere (McDonald and Ray, 1989) and has been explained as re�ecting the more robust build of the male skull which favors its preservation (�gures 4J and 4K). Nelson and Madsen (1987) observed that skulls are readily noticed by machinery operators in the gravel pits so have a better chance of being collected. �e sample of skulls of Bootherium from the Bonneville basin is one of the largest samples for the species and one of the few places where both skull morphs have been found. As is common in the western United States, mam- moth, as represented by Mammuthus columbi (�gures 4H and 4I), is more common than mastodon, Mammut americanum, and this is also true for the Bonneville ba- 187 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 Table 1. Late Pleistocene and early Holocene vertebrate taxa associated with Lake Bonneville. Compiled from Nelson and Madsen (1987), Gillette and others (1999), Broughton (2000), Grayson (2000), and Broughton and others (2000), Livingston (2000), McDonald and others (2001), Schmitt and Lupo (2016), and Wolfe and Broughton and Smith (2016). * = extinct taxa. Class Osteichthyes Gila atraria Richardsonius balteatus Catostomus ardens Catostomus discobolus Panosteus virescens Chasmistes cf. liorus cf. Salvelinus con�uentus Oncorhynchus clarki (also reported as Salmo clarki) Prosopium gemmifer Prosopium spilonotus Prosopium abyssicola Cottus bairdi Cottus extensus Class Aves Podilymbus podiceps Podiceps sp. Aechmophorus occidentalis Phalacrocorax auritus (reported as P. macropus) Branta sp. Anas sp. Athya sp. Bucephala albeola Bucephala clangula Oxyura jamaicensis Cygnus buccinator Circus cyanus Accipiter striatus Buteo sp. Falco sparverius Falco columbiarius Falco cf. mexicanus Phasianidae Rallus limnicola Porzana carolina Porphyra/Gallinula/Fulica Recurvirostra americana Phalaropus sp. Larus sp. Zenaida macroura Coccyzus americanus Tyto alba Otus sp. Bubo virginianus Glaucidium gnoma Athene cunicularius Asio sp. Ageolius acadicus Chordeiles cf. acutipennis Phalaenontilus nuttallii Melanerpes lewis Sphyrivicus sp. Picoides sp. Colantes auratus Tyrannus verticalis Eremophilia alpestris cf. Stelgidopteryx serripennis Cyanocetta stelleri Aphelocoma coerulescens Pica pica Troglodytidae Myadestes townsendi Turdus migratorius Oreoscoptes montanus Bombycilla garrulous B. cf. cedrorum Lanius ludoviciana Piranga ludociviana Pheuticus cf. melanocephalus Piplio cf. chlorurus Piplio cf. erythrophthalmus Agelaius phoeniceus Sturnella neglecta Xanthocephalus xanthocephalus Fringillidae Carpodacus mexicanus Carduelis tristis Coccothraustes vespertinus Class Mammalia Megalonyx je�ersonii * Brachylagus idahoensis Lepus californicus Lepus townsendii Sylvilagus cf. audubonii Sylvilagus cf. nuttallii Ammospermophilus lecurus Tamias minimus Marmota �aviventris Spermophilus mollis �omomys bottae �omomys talpoides Chaetodipus formosus Dipodomys microps Dipodomys ordii Microdipodops megacephalus Perognathus longimembris Perognathus parvus Lemmiscus curtatus Microtus sp. Neotoma cinerea Neotoma lepida Ondatra zibethicus Onychomys leucogaster Peromyscus sp. Pitymys sp. Reithrodontomys megalotis Vulpes vulpes Vulpes velox Canis latrans Canis lupus Arctodus simus * Ursus americanus Lynx rufus Mustela erminea Mustela frenata Mustela vison Spilogale putorius Taxidea taxus Equus sp. Platygonus compressus * Camelops hesternus * Odocoileus hemionus Navahoceros cf. fricki * Antilocapra americana Ovis canadensis Bison cf. antiquus * Bootherium bombifrons (including Symbos cavifrons) * Mammut americanum * Mammuthus columbi * 188 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 sin with 17 documented records of mammoth and only one record of mastodon (Nelson and Madsen, 1987; Larson, 1999). �is may simply re�ect di�erences in preferred habitat with mastodons more closely associat- ed with coniferous forests that would have been present at higher elevations in the Wasatch Range outside the lake basin (Miller, 1987), while the mammoths would have grazed in the more open grassland habitat at lower elevations. �e recovery of a mammoth from a gravel pit near Newcastle in the Escalante Valley, Iron Coun- ty, is the southernmost record of this taxon associated with Lake Bonneville (Larson, 1999). �e specimen was recovered from an alluvial-fan complex and was radio- carbon dated at 28,670 ± 260 14C yr B.P., which predates the lake’s highstand in that valley 12,000 years later. However, while the southern edge of Lake Bonneville did extend into the northern part of the Escalante Val- ley (Currey, 1982) the lake did not extend as far south as the site where the mammoth was found. Another taxon associated with Lake Bonneville that is relatively rare in the western United States is the �at-headed peccary, Platygonus compressus (McDonald, 2002). �e single record, consisting of most of the skele- ton of one individual comes from the Cache Valley. �e specimen was radiocarbon dated at 11,340 + 50 14C yr B.P. and indicates the �oor of the valley was inhabited by terrestrial fauna relatively soon a�er the �ood and drop in lake level. �e extinct camel, Camelops, which is uncommon, has been referred to C. hesternus, primarily based on size (�gure 4A). Mule deer, Odocoileus hemi- nous, is known only from a humerus and femur recov- ered the same gravel pit. Horse, currently not referred to species, is also present but also uncommon (�gures 4D and 4E). Among the most common species recovered from sediments associated with Lake Bonneville is the moun- tain or bighorn sheep, Ovis canadensis (Stokes and Condie, 1961; Stokes, 1966). �is species is represented by a least two dozen individuals but is a highly-biased sample. �e mountain sheep is primarily represented by crania (�gures 4F and 4G) or horn cores of adult males. Skulls of females and juveniles have not been recovered and only a very few post-cranial bones have been found. Stokes (cited in Nelson and Madsen, 1987) suggested that the large sinuses in the male skulls and horns al- lowed them to partially �oat and thus be transported. As noted by Nelson and Madsen (1987) almost all the specimens show abraded surfaces suggestive of being rolled on the beach prior to burial. Unfortunately, the way most vertebrate specimens associated with Lake Bonneville are recovered during commercial gravel and sand quarrying has precluded the recovery of any good taphonomic data. �e sample is highly biased towards larger specimens that are more easily noticed and no longer in situ. Consequently most �nds are of single bones, although a few associated bones of an individu- al are occasionally found, such as the short-faced bear which includes a partial vertebral column of posterior thoracic to sacral vertebrae, pelvis, and bones of the hind limbs (Nelson and Madsen, 1983). As expected only a few carnivores have been recov- ered from the gravel deposits. �ese include two extant canids, Vulpes vulpes and Canis lupus, and two bears, the extant Ursus americanus and extinct Arctodus simus (Nelson and Madsen, 1983, 1986, 1987) (�gure 4C). �e Arctodus is one of two records of this taxon in Utah; the other is from the Huntington Dam Mammoth Site in the Wasatch Plateau south of the Bonneville basin (Gil- lette and Madsen, 1992). �e only partial skeletons recovered to date are from the short-faced bear, Arctodus, and the sloth, Meg- alonyx. Unlike most of the other taxa, which are from deposits associated with the Bonneville shoreline, the two sloth records are from the Provo level. Also, both sloth specimens come from the south end of the basin from Point of the Mountain and Orem, and both were recovered in situ. �e sedimentologic and taphonom- ic data for the Point of the Mountain specimen suggest that the sloth carcass may have washed into Lake Bon- neville relatively intact. �e carcass was transported some distance along the Wasatch Front by longshore dri�. As the carcass was transported toward Point of the Mountain, it made its way to the edge of the wave- built terrace of a large spit. �e spit would have been migrating to the west, but the longshore current and direction of the spit would have produced foreset beds oriented to the northwest. �e carcass eventually fell o� the wave-built terrace of the spit and settled into deep- er water (approximately 10 m) that had been receiving only suspended-load, hemipelagic sediments. Eventual- 189 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 ly, foreset lamination from spit progradation buried the carcass, thus preventing scavenging and disassociation of the skeleton (McDonald and others, 2001). �e sand and gravel in which the Orem skeleton was buried are either deltaic deposits of the Provo phase of the Bonne- ville lake cycle or stream alluvium related to the Provo phase of the Bonneville lake cycle (Machette, 1992). �e skeleton was in coarse, ungraded and unbedded gravel, but with slight imbrication of the gravel clasts immedi- ately adjacent to the bones that suggested high-energy �uvial sedimentation. Virtually all of the small mammal remains associat- ed with Lake Bonneville have been recovered from cave deposits. Most of these accumulations appear to be the result of foraging by owls. �e small mammal remains from Homestead Cave have been intensively studied (Grayson, 2000) based on 183,798 identi�ed bones and teeth. �is documentation of the changing proportions of species pairs and species replacement in succeeding strata indicates major shi�s in the climate and conse- quently the vegetation (Grayson, 2000). Two species of Dipodomys are present, D. microps and D. ordii. D. microps, the chisel-tooth kangaroo rat, which uses its lower incisors to shave o� the outer hypersaline tissue from the leaves of Atriplex sp. in order to access the pal- atable inner tissue, whereas D. ordii is a granivore that is usually associated with sagebrush habitat, so there is minimal overlap in the distribution of the two species. �e increase in the number of specimens of D. microps and decline of D. ordii in the stratigraphic sequence sug- gests the progressive replacement of sagebrush-domi- nated habitat by shadscale-dominated habitat through the Holocene. A similar pattern is demonstrated by the changes in abundance of Microtis and Lemmiscus. Mi- crotis utilizes grassland habitat whereas Lemmiscus pre- fers habitat dominated by sagebrush, usually Artemisia tridentata. Microtis is common in the late Pleistocene and early Holocene sediments at Homestead Cave but decreases in number a�er 8.3 ka and is uncommon a�er 7 ka, and is replaced by Lemmiscus as the common vole. Also, around 8.3 ka, Neotoma lepida replaces N. cine- rea in the fauna. A similar pattern is seen in the pock- et mice, Perognathus, with P. parvus common in the late Pleistocene and then replaced by P. longimembris. P. parvus in the Great Basin today is found at higher elevations indicating a preference for cooler tempera- tures whereas P. longimembris tends to be found at low- er elevations with higher temperatures. Likewise, the pocket gopher, �omomys talpoides, is found at higher elevations and mountain valleys in Utah today while T. bottae lives in the lower valleys. At Homestead Cave, T. talpoides is present only in the late Pleistocene and is replaced by T. bottae, indicating a transition from cool- er to warmer temperatures. �e yellow-bellied marmot, Marmota �aviventris, lives at higher elevations in the southern part of its range and at lower elevations in the more northern parts of its range, re�ecting its sensitiv- ity to warmer temperatures. At Homestead Cave, it is more common in the lower late Pleistocene strata and appears to have disappeared from the area around 8 ka so its disappearance may be indicative of warming tem- peratures. Most bird remains associated with Lake Bonneville consist of isolated �nds and the only known avifaunas are from cave deposits, like those of Homestead Cave. �e avifaunal assemblage from Homestead Cave con- sists of 6000 specimens representing 75 species from 26 families (Livingston, 2000). �e avifauna includes not only taxa that would be expected to be associated with a lake—divers, specialized �shers, waterfowl, shorebirds and marshbirds—but also upland game birds, wood- peckers, and perching birds. �e diurnal predators like hawks and falcons, and nocturnal predators, owls, are believed to have been the primary contributors to the fauna in the cave. �e three most common families of birds from Homestead Cave are the Podicipedidae (grebes) at 30% of the total assemblage, Anatidae (ducks) at 15% of the assemblage, and Alaudidae (larks) at 20% of the assemblage. �e presence of the two aquatic families is expected given the cave’s association with the lake. In fact, the two lowest strata in the cave, when the lake lev- el was still relatively high, are dominated by waterfowl in terms of both the number of taxa, and percentage of bones from waterfowl recovered. Larks do not become common until the upper stratigraphic levels, re�ecting the lower lake level and presence of more open habi- tat. Missing from the avifauna are larger taxa like loons, pelicans, swans, cranes, and herons that are all larger than the predatory birds in the fauna. Cormorants and 190 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 mergansers, which are obligate piscivores, are restricted to the lower levels of Homestead Cave and disappear early, likely re�ecting the loss of a food source with low- er lake levels and increased salinity. While �sh do not live in Great Salt Lake today, fresh- er water during the Bonneville lake cycle supported a variety of �sh (Broughton and Smith, 2016). Brough- ton and Smith (2016) described 10 primary localities in the Bonneville basin that produced �sh remains: six are open-air sites and four are caves. Broughton and Smith (2016) documented 13 species of �sh represented by fossils, out of the 21 species present in the extant fauna (table 1). �e �sh found in Lake Bonneville sediments account for three of the four endemic and �ve of the nine native species in modern Bear Lake (�gure 3). Homestead Cave is in the Lakeside Mountains (�g- ure 3). �e opening of the cave at 1406 m is between the Provo and Stansbury shorelines. �e Pleistocene fauna recovered included 11 species of freshwater �sh con- centrated in the lowest stratum of the deposits and 11 freshwater species from sediments that accumulated in the cave a�er 11,200 14C yr B.P. Eight of the species oc- cur in Bear Lake. �e �sh remains are thought to have been caught by owls (Broughton, 2000; Broughton and others, 2000). Analysis of the 87Sr/86Sr values of the �sh bone from the lowest stratum of the cave suggests they grew near the terminal Pleistocene Gilbert shoreline. A decrease in �sh size and an increase in species tolerant of higher salinities or temperatures in the lowest depos- its suggest multiple die-o�s associated with declining lake levels. An initial, catastrophic, post-Provo die-o� occurred at 11,300 –11,200 14C yr B.P. (~ 13.2 to 13.1 cal ka) and was followed by at least one rebound or recol- onization of �sh populations. Around 3.7 cal ka there was a highstand of the lake and this is re�ected in a peak in the frequency of bones of Gila atraria (Utah chub) in the cave sediments. Despite this rebound �sh were gone from Lake Bonneville sometime before 10,400 14C yr B.P. Cathedral Cave which is close to Homestead Cave has a distinctively di�erent �sh fauna that consists exclusively of sculpin. As sculpins prefer cold deep wa- ter their presence in the cave sediments is attributed to accumulating during a deep-water phase, ca. 100–200 m of Lake Bonneville when the lake was at either the Bonneville or Provo level. Strata above the level with the sculpins had a radiocarbon age of 15,310 ± 60 14C yr B.P. (Broughton and Smith, 2016). ROAD GUIDE �e road guide begins at the Grand America Hotel in Salt Lake City (�gure 5). �e route heads south to the G.K. Gilbert Geologic View Park, the Draper spit, Steep Mountain shorelines, the Point of the Mountain spit, and the American Fork delta. �e route then heads west to the Stockton Bar, north to Great Salt Lake, and east to return to Grand America. Mileage (mi) Description Interval / Cumulative 0.0 / 0.0 Grand America Hotel’s porte cochere entrance at approximately 60 East 600 South Street, Salt Lake City (�gure 5). Turn le� out of porte cochere and proceed several hundred feet to State Street. 0.0 / 0.0 Turn right onto State Street. 0.6 / 0.6 Turn right onto 900 South and proceed west. 0.3 / 0.9 Turn le� onto West Temple and merge onto the Interstate 80 (I-80) East ramp to Chey- enne. 2.5 / 3.4 Merge onto I-80 East. 4.6 / 8.0 Take exit 128 for I-215 South. 6.3 / 14.3 Take exit 6 for 6200 South and turn le�, pro- ceeding east. 0.8 / 15.1 6200 South curves right (south) and becomes Wasatch Boulevard. 3.2 / 18.3 Bear right at signal to continue on Wasatch Boulevard. 1.1 / 19.4 Turn right onto Little Cottonwood Road at signal. 0.1 / 19.5 To Stop 1. Pull into dirt parking area on right at 3345 East Little Cottonwood Road, Sandy. Stop 1 G.K. Gilbert Geologic View Park Located near the mouth of Little Cottonwood Can- yon, this geologic park showcases multiple geomorphic and geologic features and has long been a destination for �eld trips. �e park was dedicated in 2008 and in- 191 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 cludes �ve non-technical interpretive signs with infor- mation about bedrock geology, mining and quarrying activity, the Wasatch fault, Lake Bonneville, and glaci- ation (�gure 6). �ree bedrock formations are visible from the park: (1) Early Proterozoic Little Willow Formation, (2) Late Proterozoic Big Cottonwood Formation, and (3) Oligo- cene Little Cottonwood stock (Eldredge, 2008). Con- torted quartz schist and gneiss primarily comprise the Little Willow Formation. Alternating shale and quartz- ite originally deposited in a tidal/shoreline environment generally comprise the Big Cottonwood Formation. Quartz monzonite comprises the bulk of the Little Cot- tonwood stock. Gold was mined from the Little Willow Formation more than a century ago, and mine dumps are evident near the north side of the canyon mouth (Eldredge, 2008). “Granite” (quartz monzonite) has been quarried from the Temple Quarry, located just out of view in the lower canyon. �is stone was used to construct the Church of Jesus Christ of Latter-day Saints’ Salt Lake Temple and other prominent buildings in Salt Lake City and has been quarried intermittently since the 1860s. Formed by repeated earthquakes and reaching more Figure 5. Field trip stops. �e blue line is the 5200-foot-elevation contour, which approximates the Bonneville shoreline (not adjusted for isostatic rebound). 192 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 than 30 m high, the scarps visible here on the Salt Lake City segment are some of the largest of the Wasatch fault zone (�gure 7). �e Wasatch fault zone is an ap- proximately 370-km-long, 10-segmented normal fault system that forms the Wasatch Front and the eastern boundary of the Basin and Range Province (Machette and others, 1992). Faulting began approximately 18 Ma (Parry and Bruhn, 1987) and vertical slip rates have ap- parently varied through time. Long-term slip rates are about 0.7 mm/yr since ca. 18 Ma (Parry and Bruhn, 1987), 0.2–0.4 mm/yr since 5 Ma (Armstrong and oth- ers, 2004), and 0.5–1.0 mm/yr since 0.4–0.8 Ma (Mayo and others, 2009). Displaced latest Pleistocene shore- lines and fans yield vertical slip rates of approximately 0.1–0.3 mm/yr (Machette and others, 1992). Post-Pro- vo level paleoseismic trenching data yield slip rates of approximately 1–2 mm/yr (e.g., Machette and others, 1992; Lund, 2005; DuRoss and others, 2016). Modern geodetic horizontal extension across the fault zone is ~1.6–3.2 mm/yr (Chang and others, 2006; Kreemer and others, 2010). �e park is located above the elevation of the Bonne- ville shoreline and lake features are not visible. �e park rests on glacial till and would have been covered by glacial ice near the time of the Bonneville highstand at 18 ka (Laabs and Munroe, 2016). Glacial features visible from the park include the classic U-shaped valley of Little Cottonwood Canyon, moraines, and scattered glacial boulders. �is is one of only a few localities in the world where mid-latitude alpine glaciers entered lakes during the Last Glacial Maxi- mum. As discussed in the Climate and Glaciation section above, Laabs and Munroe (2016) dated erratic boulders found both on the north side of the canyon mouth and on top of the moraine crest visible to the south, which yielded Figure 6. Annotated view to the northeast from the G.K. Gilbert Geologic View Park. 193 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 a cosmogenic exposure age of ca. 18 ka (�gures 6 and 7). Continue road guide. 0.0 / 19.5 Turn le� out of parking area onto Little Cot- tonwood Road. 0.1 / 19.6 Turn right onto Wasatch Boulevard and proceed southwest on Wasatch Boulevard. 4.4 / 24.0 Turn le� onto 1700 East and proceed south. 0.7 / 24.7 1700 East curves right (west) and becomes Draper Parkway. 0.4 / 25.1 To Stop 2 at 1360 East Draper Parkway, Draper. Stop 2 Walmart Neighborhood Market �is is intended as a restroom stop, but the parking lot provides distal views to the south of Steep Moun- tain (�gure 8) and the entire north �ank of the Traverse Mountains (see Stop 3). �is store and parking lot are on the south �ank of the Draper spit. �e bare slope across the road to the north was the headwall of a now reclaimed gravel pit. �e sand and gravel that comprise the spit were presumably sourced from Little Cotton- wood and Big Cottonwood Canyons to the north. �e elevation of the parking lot is below the Provo shoreline at approximately 1420 m. Continue road guide. 0.0 / 25.1 Turn le� out of parking lot onto Draper Parkway and proceed west. 0.2 / 25.3 Turn le� onto 1300 East at signal. 0.2 / 25.5 Proceed through tra�c circle and continue on 1300 East. 1.6 / 27.1 Turn right onto Highland Drive and pro- ceed southwest. 2.2 / 29.3 To Stop 3. Pull o� on the right shoulder south of intersection of Highland Drive and Bangerter Parkway-Traverse Ridge Road (approximately 155 East Highland Drive, Draper). Stop 3 View of Steep Mountain �e Traverse Mountains are an east-west trending range in an area dominated by north-south trending ranges. �ey extend between the Wasatch Range to the Figure 7. View to the southeast from the G.K. Gilbert Geologic View Park. Prominent ridge at center is a le� lateral glacial mo- raine that is o�set by two prom- inent west-facing Wasatch fault scarps (yellow lines). 194 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 east and the Oquirrh Mountains to the west, and have a gap cut by the Jordan River. Additionally, the Traverse Mountains lie at the boundary between two segments of the Wasatch fault, the Salt Lake City segment to the north and the Provo segment to the south. �e location and orientation of the Traverse Moun- tains is thought to be due to an ancient crustal struc- ture that predates the crustal extension that created the modern landscape. �e east-west orientation of these mountains subjected their north �ank to intense waves generated across a 200-km-long, unimpeded stretch of Lake Bonneville that once extended to the north (Scho- �eld and others, 2004). �e north-facing Steep Mountain part of the Tra- verse Mountains is composed of highly fractured quartzite of the Pennsylvanian Oquirrh Group (�gure 9). �is quartzite was readily pulverized by waves, cre- ating an enormous supply of sand and gravel. Beneath the steep face of bedrock lies the Steep Mountain beach complex, a broad Bonneville-level depositional plat- form consisting of sediment deposited o�shore as the lake transgressed to its highstand (Oviatt and Jewell, 2016). �e sediment-to-bedrock contact below this platform is evidenced by a change in vegetation on the slope above the Provo shoreline. �e sand and gravel of the upper half of this slope is dominated by shrub- by vegetation, the bedrock of the lower half by grasses (�gure 8). While substantial amounts of sand and gravel remained in the Steep Mountain beach complex, strong longshore currents traveling west and south through the Jordan River Narrows transported much of the ma- terial, redepositing it in the Point of the Mountain spit (Scho�eld and others, 2004). Previous workers (e.g., Shelton, 1966) have cited this location as a prominent example of a Bonneville shore- line erosional platform, presumably developed during a prolonged period (hundreds of years or more) of open basin over�ow at the Bonneville highstand. However, Oviatt and Jewell’s (2016) recognition of a deposition- al platform deposited during lake transgression shows that there is no evidence for such a period of prolonged over�ow at this location. �is interpretation coupled with consistent evidence from other locations suggests that the lake transgressed to the Bonneville shoreline and then over�owed only brie�y before the Bonneville �ood, a return to Gilbert’s original 1890 hypothesis. Continue road guide. 0.0 / 29.3 Continue southwest on Highland Drive. 0.6 / 29.9 Turn le� onto Minuteman Drive and pro- ceed south. Note, Minuteman Drive be- comes Frontage Road. 2.7 / 32.6 Turn le� onto Flight Park Road and proceed east. 0.9 / 33.5 Flight Park Road curves le� and loops around to the west. 0.6 / 34.1 To Stop 4. Stop at end of Flight Park Road. Stop 4 Flight Park State Recreation Area, Utah County �e Point of the Mountain spit and adjacent Steep Mountain beach are two of the largest and most spectac- ular shoreline features of the Bonneville basin. As with all prominent shoreline features in the Bonneville basin, G.K. Gilbert (1890) was the �rst to document this spit complex (�gure 10). �e gross shape of the spit is that of Figure 8. Aerial view (toward west) of the prominent Bonneville- and Provo-level shorelines at Steep Mountain on the north side of the Traverse Mountains. Photo courtesy of Mark Bennett (una�liated). 195 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 a V-shaped embankment anchored laterally to bedrock, but in reality it is a complex feature that formed not only during the Bonneville lake cycle but also during known older lake cycles (Scho�eld and others, 2004). Based on clast lithology and geomorphic analysis of multiple shoreline features at and adjacent to Point of the Mountain, Scho�eld and others (2004) determined that the Point of the Mountain spit formed as a result of the highly fractured bedrock of Steep Mountain be- ing exposed to wave trains that approached from the north-northwest causing north-to-south longshore sed- iment transport (�gure 11). Shoreline development and sediment transport on the southern portion of the spit were minimal. A predominant northerly wind direction during the Pleistocene matches modern winter storms that tend to come from the northwest, though southern winds from prefrontal lows can also be signi�cant. Point of the Mountain has been extensively mined for aggregate (�gures 12 and 13). �e total amount of sand, gravel, and rock removed is unknown, but according to Utah Division of Oil, Gas and Mining documents, rough- ly 25 million tons was mined just from 2009 to 2014. Just east and uphill from this stop a mine is permitted with a 5 to 22 year life expectancy and an estimated 2 million tons of mostly bedrock material removed per year. Continue road guide. 0.0 / 34.1 Return back down Flight Park Road to Frontage Road. 1.5 / 35.6 Turn le� onto Frontage Road. Note, Front- age Road becomes Digital Drive. Proceed east and then south. 2.1 / 37.7 Turn le� onto Timpanogos Highway Figure 9. Prominent Bonneville bench and Provo-level shoreline at Steep Mountain, on the north side of the Traverse Moun- tains. Figure 10. Point of the Mountain “V-shaped embankment” (spit). North is toward top of page. From Gilbert (1890, plate 7). 196 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 Figure 11. Shaded-relief image of the Traverse Mountains. Image is approximately 5 km across. Figure 12. View of the north side of the Traverse Mountains. Photo shows Point of the Mountain before any signi�cant min- ing activity; date unknown. Figure modi�ed from Shelton (2004). 197 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 State Road (SR) 92 and proceed east. 6.0 / 43.7 Turn le� onto North County Boulevard (4800 West) and proceed north. 0.2 / 43.9 Turn right onto 11200 North and pro- ceed east to end of road. 0.3 / 44.2 To Stop 5, 4560 West 11200 North, Highland City. Stop 5 American Fork Delta �e following section is modi�ed from Godsey and others, 2005a. �e American Fork delta is a classic Gilbert-type delta. Gilbert’s 1890 study of Pleistocene Lake Bonne- ville was the �rst detailed geomorphic and stratigraphic study of gravelly deltas. Gilbert is reported to have vis- Figure 13. Mining has removed much of the Pleistocene sediments and some of the bedrock at Point of the Mountain. Top photos 1993 (le�) and 2015 (right) Google Earth imagery. Bottom photo of gravel pit shows view to the north from Flight Park State Recreation Area in Utah County. Google Earth imagery © 2015 Google Inc.; top le� image—U.S. Geological Sur- vey and top right image–Landsat. 198 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 ited all of the lake’s deltas; however, only two locations were discussed in detail, the Bonneville-level delta at American Fork and the Provo-level Logan River del- ta (located in Cache Valley, northeastern Utah). From these observations of the lake’s coarse-grained deltas, Gilbert developed the topset-foreset-bottomset model (�gure 14). However, gravel-pit exposures show that one of Gilbert’s original study localities, the Bonneville-lev- el delta at American Fork, is composed almost entirely of subhorizontal gravel (topsets). Taking advantage of such gravel-pit exposures not available to Gilbert, this model can be re�ned to show two end member deltas: (1) topset-dominated deltas deposited during the Bon- neville transgression, and (2) foreset-dominated deltas deposited at the Provo shoreline and during the Pro- vo regression (�gures 15 and 16) (Milligan and Chan, 1998). Other gravel-pit exposures at the Bonneville lev- el of the Big Cottonwood Canyon and American Fork deltas displayed a predominance of horizontally strat- i�ed gravel that comprises the topset-dominated delta system. Other gravel-pit exposures at the Provo level of the Big Cottonwood Canyon and Brigham City deltas displayed a predominance of steeply dipping gravel that comprises the foreset-dominated delta systems. �ree key factors contribute to the development and depositional styles of these Wasatch Front Gilbert del- tas: active tectonism, rapid lake-level �uctuation, and drainage basin deglaciation. Slip on the Wasatch fault zone produced the steep drainage basins responsible for the overall coarse-grained nature of these Gilbert deltas. Figure 14. Gilbert’s classic topset-foreset-bottomset model of coarse-grained deltas (from Gilbert, 1890). 199 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 However, slip rates of 0.76–1.5 mm/yr, the average for the last 15 kyr in study areas (Machette, 1988; Schwartz and Lund, 1988; Personius and Scott, 1992), are over- printed by lake-level change that can exceed 80 mm/yr (long-term post-Provo regression rate) or even 125 m/ yr for the nearly instantaneous Bonneville �ood event. Drainage basin deglaciation and the release of gla- cial outwash played a role in sediment supply and, thus, the distribution of facies found at some localities. �e e�ects of deglaciation are best seen at Big Cottonwood Canyon (about 5 km north of Stop 1), where glaciers neared the Bonneville shoreline producing topsets of subaerial glacial outwash. Evidence for glaciation (e.g., moraines and striations) is also found in the upper reaches of the American Fork drainage basin (Laabs and others, 2011). Facies distributions were most strongly in�uenced by lake level �uctuation, which largely controls accom- modation space and sediment supply. Topset-dominat- ed deltas formed with increasing water depth created by climate-driven transgression to the Bonneville shore- line. Foreset-dominated deltas formed with decreasing water depth. Catastrophic lake-level drop due to the Bonneville �ood and the subsequent climate-driven Provo regression not only greatly reduced accommoda- tion space, but also provided abundant sediment supply by exposing unlithi�ed Bonneville-level deltaic sedi- ments for reworking. �e Bonneville-level American Fork delta seen at this stop exempli�es a topset-dominated system that dis- plays a classic “∆” shape in plan view. �ese gravel topset deposits consist of horizontal clast-supported pebble and cobble gravel with lenses of silty sand deposited during the Bonneville transgression and brief highstand. Figure 15. Top photo shows the horizontal gravel of topset-dominated at the Bonneville level (late-transgressive-phase), American Fork delta, ca. 1993. Bottom photo shows the steeply dipping gravel of a foreset-dominated delta, Provo level at Brigham City, ca. 1993. 200 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 Near the top of the delta (dark band in top photo of �gure 15) is an intervening 9-m section of sandy clay and coarse-grained to very coarse grained sand with granules and oblate pebbles. Sedimentary structures include wave ripples and tabular cross-bedding. �e cross-bedding suggests a southerly �ow direction (par- allel to the shoreline) and is likely to have been created by littoral currents. �e presence of oblate pebbles and symmetric ripples suggest shallow-water, wave-in�u- enced deposition in a delta-front beach environment. �e occurrence of this �ne-grained beach facies amidst coarse-grained delta topsets may be attributed to a downward lake-level oscillation (Machette, 1988). �e drop in lake level during this oscillation (�gure 1) probably caused the American Fork River to incise a channel through the delta, thus transferring the river deposition westward into the basin (�gure 17). �is riv- er channel (until �lled) would have cut o� the coarse- grained sediment supply (during the oscillation rise and �nal transgression to the Bonneville shoreline), allow- ing the accumulation of the �ner-grained beach and delta-front sands. Continue road guide. 0.0 / 44.2 Return on 11200 North to 4800 West (North County Boulevard). 0.3 / 44.5 Turn le� onto 4800 West and proceed south. 0.2 / 44.7 Turn right onto SR 92 and proceed west. 5.0 / 49.7 Turn le� onto 1200 West and proceed south. 1.4 / 51.1 1200 West curves right and becomes SR 85, proceed west. 2.8 / 53.9 Turn le� onto Redwood Road and proceed south. 1.8 / 55.7 Turn right onto SR 73 and proceed west. 0.9 / 56.6 Turn right to stay on SR 73 and continue west. 35.7 / 92.3 Turn right onto SR 36 towards Stockton and Tooele and proceed north. 5.2 / 97.5 Turn right onto Silver Avenue (look for Sin- clair Station on the corner) and proceed east. 0.2 / 97.7 Silver Avenue curves right and becomes Copper Street. Proceed south. 0.2 / 97.9 Turn right onto Roger Street and proceed west. 0.1 / 98.0 To Stop 6. Figure 16. Schematic representation of American Fork’s topset-dominated del- ta (area above “TST/HST” on top �g- ure) deposited during the Bonneville transgression and brief highstand, and Brigham City’s foreset-dominated del- ta (lower �gure) deposited at the Provo shoreline and during the Provo regres- sion. Sequence stratigraphy terms: SB = sequence boundary, LST = lowstand sys- tems tract, TST = transgressive systems tract, HST = highstand systems tract. From Milligan and Chan (1998). 201 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 Stop 6 Alex Baker Baseball Park (382 South Roger Street, Stockton) �is was the planned lunch stop. �e south side of the Stockton Bar and spit complex can be viewed from here. See Stop 7 for a complete description. Continue guide. 0.5 / 98.6 Return to SR 36, turn right, and proceed north. 1.9 / 100.5 Turn le� onto entrance road for Bauer Pit and park. 0.0 / 100.5 To Stop 7 at the Bauer Pit entrance road. Stop 7 Stockton Bar Overview, Tooele County �e following section is modi�ed from Godsey and others, 2005a. General Description �e Stockton Bar is enormous and perhaps the most impressive geomorphic feature found in the Bonneville basin. G.K. Gilbert �rst documented this area in the 1890 monograph (�gures 18 and 19). Stockton Bar consists of a series of spits, barriers, and beach ridges in the valley between the Stansbury and Oquirrh Mountains. �e bar and spit complex ulti- mately isolated lake waters in Rush Valley to the south from the main body of Lake Bonneville (Gilbert, 1890; Gilluly, 1929; Burr and Currey, 1988). How the barri- er bar and spit complex grew and evolved has still not been satisfactorily resolved (C.G. Oviatt, Kansas State University, written communication, 2016). Complex surface geomorphology and internal stratigraphy has led to various interpretations of its development. Gilbert (1890) recognized a series of seven sequen- tial “beds” between the Oquirrh Mountains and South Mountain that was correctly interpreted as building during the lake’s rise to the Bonneville level and creat- ing the Stockton Bar (�gure 18). Gilbert also recognized that the Stockton spit complex and an unnamed spit on the west side of Stockton Bar superseded this barrier bar series. Furthermore, Gilbert recognized that buried stratigraphy might further re�ne or reinterpret the sim- ple depositional history. Based on detailed study of surface geomorphology, Burr and Currey (1988) show transgressive-age shore- lines (T on �gure 20) to the north and south of the main bar, and a series of shorelines (B1, B3, B5, B6, B8 on �g- ure 20) formed during a prolonged period of over�ow at the Zenda threshold. However, there is no indepen- dent evidence of prolonged over�ow at the Bonneville highstand (C.G. Oviatt, Kansas State University, written communication, 2016). Based on ground-penetrating radar, Smith and oth- ers (2003) believed the barrier bar formed �rst by ver- tical accretion during slow transgression due to climate or basin subsidence, followed by the continued trans- gression and a reorientation of longshore transport that Figure 17. Aerial view of the Bonneville-level (late-transgres- sive-phase) American Fork delta ca. 1970. Note incision by the modern American Fork river channel. Photo from Utah Geological Survey aerial image collection. 202 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 Figure 18. G.K. Gilbert recognized a series of seven sequential “beds,” labeled a–g, deposited during the lake’s rise to the Bonneville level. Gilbert also postulated that the Stockton spit complex and an unnamed hook-shaped spit on the west side of Stockton Bar superseded this barrier bar series. Map of Stockton Bar by H.A. Wheeler as illustrated in Lake Bonneville, U.S. Geological Survey Monograph 1, by G.K. Gilbert (1890). 203 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 created Stockton spit, and a �nal rise that again reori- ented longshore transport and created the smaller up- per spit (�gure 21). �ese various interpretations are likely to be re�ned by future work. �e Stockton Bar as a Geoantiquity Concerned scientists have been involved in research to document and preserve important landforms, named geoantiquities or geosites, related to Lake Bonneville and Pleistocene Earth surface processes (Chan and others, 2003; Chan and Godsey, 2016). Sediments that make up Lake Bonneville landforms are typically well rounded, well sorted, and unconsolidated, making them prime aggregate material and targets for quarrying operations. �e Stockton Bar tops the list of important geoantiqui- ties, not only because of its scienti�c and educational merit, but also because of its historical, aesthetic, and recreational value. �e bar has been a site of mining ac- tivity for at least 60 years, but excavation e�orts have increased steadily with the growing aggregate needs of neighboring urban communities. A�er many public speeches, �eld trips, communi- ty education campaigns, and partnerships with various conservation organizations, the struggle for preserva- tion of the Stockton Bar has met with mixed but gener- ally positive results. A land�ll and a tailings pond exist on the north side of the bar, homes have been developed on the transgressive shorelines to the south, and min- ing still threatens to remove deposits on the east side of the bar. However, multiple attempts for new mining permits have been stalled. E�orts to protect the bar began in earnest in 1999 with a permit request to remove 400,000 tons of sand and gravel. Due to extensive e�orts by local citizens and scientists, the Tooele County planning commission did not issue the permit. In 2009, an uprising of local citizens supported by scientists prevented rezoning that would have allowed increased mining. Again in 2015, under pressure from local citizens and nationally and internationally renowned scientists, the Tooele County planning commission rejected a rezoning request by an aggregate mining company. Perhaps most signi�cantly, the Tooele County Gen- eral Plan Update 2016 includes language that recog- nizes the Stockton Bar as “perhaps the most important natural feature in the area,” the mining of which would be “an incalculable loss,” and recommends modifying County Code “to ensure the Stockton Bar and other ir- replaceable natural features are preserved and protected in perpetuity.” While encouraging for preservationists, the landownership of Stockton Bar remains private and thus protection is uncertain. Continue north on SR 36. Continue road guide. 16.2 / 116.7 Take the I-80 eastbound ramp. 5.4 / 122.1 Take exit 104 for SR 202. Figure 19. Top image, “�e Great Bar at Stockton, Utah” as illustrated in Lake Bonneville, U.S. Geological Survey Mono- graph 1, by G.K. Gilbert (1890). �e name has subsequently been shortened to Stockton Bar. Bottom image shows a simi- lar but recent perspective of the Stockton Bar. Photo courtesy of Holly Godsey (University of Utah). 204 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 0.5 / 122.6 Turn le� onto SR 202 and proceed northwest. 0.3 / 122.9 Turn le� onto frontage road and proceed southwest towards Great Salt Lake State Park. 2.0 / 124.9 To Stop 8 at the Great Salt Lake Marina, 1075 South 13312 West, Magna. Stop 8 Great Salt Lake State Park Marina Although Great Salt Lake is but a small remnant of Lake Bonneville, it has the largest surface area of any natural lake west of the Mississippi River. As a terminal lake with no outlet, its level is in constant �ux (�gures 22 and 23). In Holocene time its level has �uctuated less than 15 m. In the roughly 170-year historic period the lake has �uctuated about 6 m, responding primarily to changes in precipitation and consumptive use of fresh water in the basin. Construction of causeways and dikes has divided the lake into four major parts (north arm, south arm, Bear River Bay, and Farmington Bay) with Figure 20. Based on detailed study of surface geomorphology, Burr and Currey (1988) show transgressive-age shorelines (T) to the north and south of the main bar, and a series of shorelines (B1, B3, B5, B6, B8) that formed during a prolonged peri- od of over�ow at the Zenda threshold. However, there is no independent evidence of prolonged over�ow at the Bonneville highstand (C.G. Oviatt, Kansas State University, written communication, 2016). Rp and Rg demark Rush Valley Provo- and Gilbert-age shorelines. Modi�ed from Burr and Currey (1988). 205 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 di�erent levels and salinities (�gure 24). �e lake is a major source of mineral resources, supports a multi-million dollar brine shrimp industry, is a globally important migratory bird site, and much more. Entire books are dedicated to Great Salt Lake (e.g., Gwynn, 2002). �is �eld trip guide will not at- tempt to touch upon this wealth of information but will give a brief overview of microbialites recently exposed by near-record low lake levels. General Characteristics of Great Salt Lake (Gywnn, 1996 and modi�ed from Vanden Berg and others, 2015) • 33rd largest lake in the world (largest fresh or salt- water lake in the United States a�er the Great Lakes) • Averages 121 km long by 56 km wide. • Surface elevation: historical average (since 1847) ~1280 m (4200 �) covering 4185 km2, historical high 1283.77 m (4211.85 �) in 1986 and 1987, historical low 1277.52 m (4191.35 �) in 1963. Railroad Elevation (m) Dip angle Dirt road Paved road Highway Sediment transport direction St oc kt on s p it Rush Valley Tooele Valley up pe r s pi t Stockton Tooele 5 km f e d c b a 0 km 0.50 36 b arrier bar 4-10 4-10 25 25 25 O ª1583 ª 1546 ª ª 1548 ª 1585 ª 1576 ª 1594 1 2 3 Figure 21. Smith and others (2003) used ground-penetrating radar to develop a depositional model of the Stockton Bar com- plex that suggests the barrier bar portion (1) formed �rst during slow transgression, followed by continued transgression and a reorientation of longshore currents that deposited the Stockton spit (2), and a �nal rise that again reoriented currents and deposited the smaller upper spit (3). Modi�ed from Smith and others (2003). 206 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 Figure 22. Areal extent, elevation, volume, and maximum depth of Great Salt Lake at historic high, average, and historic low-water levels. From Gwynn (1996). 207 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 • Lake-level �uctuations: 0.3-0.6 m (1-2 �) annually on average. • Maximum depth at average elevation (1280 m [4200 �]): ~10 m (33 �). • Volume: 19 km3. • Salinity: south arm = 5-22% (highly dependent on lake level and location), north arm = 24–26% (near or at salt-saturation point). • Average chemical composition: chloride = 56%, sodium = 32%, sulfate = 7.0%, magnesium = 3.3%, potassium = 2.1%, calcium = 0.2%. • Estimated reduction of lake level due to consumptive uses: 3.4 m (11 �) (Wurtsbaugh and others, 2016). • Estimated south arm lake-level drop due to a 55 m long Union Paci�c causeway breach: ~0.5 m (1.5 �). Breach made on December 1, 2016. • Lake levels for October 2016: south arm = ~1277.8 m (4192.3 �), north arm = ~1276.8 m (4189.1 �), maximum water depth ~7.7 m (25 �). Fish At its historic high level in 1986 the lake’s south arm salinity dropped below 6%. �is was low enough for a breeding population of brackish-water killi�sh (Lucania parva) to enter the lake from a formerly isolated spring located on the south shore (Stephens, 1990). With the subsequent lake-level drop the killi�sh were gone by the following spring. Microbialites An unforeseen bene�t of the otherwise problem- atic recent low lake levels is the exposure of incredible expanses of microbialites (�gure 25). Microbialites are organic sedimentary deposits formed when microbial communities (cyanobacteria) trap and bind sediment and/or form the locus of mineral precipitation, prin- cipally calcium carbonate (Burne and Moore, 1987). More commonly known stromatolites are a type of mi- Figure 23. Great Salt Lake hydrograph (from Vanden Berg and others, 2015). 208 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 crobialite that exhibit internal lamination. �e microbi- alites of Great Salt Lake do not typically show internal lamination but a thrombolitic (clotted) fabric (�gure 26) (Chidsey and others, 2015). Microbialites develop in mats, typically composed of bacteria, fungi, protozoans, or algae. Lindsay and others (2016) have used DNA sequencing to determine the abundance of bacteria, archaea, and eukarya in Great Salt Lake microbialities (�gure 27). �eir results show south arm of Great Salt Lake microbialites contain an abundance of photoautotrophic taxa that may drive carbonate precipitation and thus suggest the microbial- ites are still actively growing. Figure 24. Google Earth image of Great Salt Lake. Image dates range from August 16, 2014 to July, 8, 2016. Google Earth imagery © 2015 Google Inc. 209 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 Figure 25. Microbialites northeast of Stansbury Island, south arm Great Salt Lake. �e large bioherm (lower le�) has an ap- proximate 3 m diameter. �e green-brown color on the microbialites is due to a covering of cyanobacteria. �e lighter area is dead and bleached due to exposure. Photo courtesy of Michael Vanden Berg, Utah Geological Survey. Photo date November 4, 2015. Figure 26. A sliced microbialite from south arm of Great Salt Lake. Photo courtesy of Michael Vanden Berg, Utah Geolog- ical Survey. Figure 27. Relative abundances of archaeal, bacterial, and eukaryl rRNA genes in south and north arms of Great Salt Lake. Modi�ed from Lindsay and others (2016). 210 Shorelines and Vertebrate Fauna Pleistocene Lake Bonneville, Utah, Idaho, and Nevada Milligan, M., and McDonald, H.G. Geology of the Intermountain West 2017 Volume 4 Continue road guide. 2.8 / 127.7 Return to I-80 eastbound ramp and proceed east towards Salt Lake City. 14.7 / 142.4 Take exit 121 for 600 South and proceed east on 600 South. 1.6 / 144.0 Turn le� onto State Street and proceed north. 0.2 / 144.2 Return to Grand America Hotel. END OF ROAD LOG. ACKNOWLEDGMENTS No new work is presented in this geologic road guide. It is merely a compilation of previously pub- lished work. As such, we acknowledge the authors of our main sources of information: Don Currey (Univer- sity ofUtah), Holly Godsey (University of Utah), Benja- min Laabs (North Dakota State uinversity), Jack Oviatt (Kansas State University), Marith Reheis (U.S. Geologi- cal Survey), Ian Scho�eld (Loughlin Water Associates), and Michael Vanden Berg (Utah Geological Survey). A new comprehensive book, Lake Bonneville, A Scientif- ic Update (edited by Oviatt and Shroder, 2016) neces- sitated updates to the original manuscript. We would also like to acknowledge and thank Jack Oviatt for his extremely helpful review of this manuscript. We thank the Vertebrate Paleontology Department of the Natural History Museum of Utah for permission to photograph the fossil specimens in their collection illustrated in �g- ure 4. REFERENCES Adams, K.D., and Bills, B.G., 2016, Isostatic rebound and palin- spastic restoration of the Bonneville and Provo shorelines in the Bonneville basin, UT, NV, and ID, in Oviatt, C.G., and Shroder, J.F., Jr., editors, Lake Bonneville—a scienti�c update: Cambridge, Elsevier, p. 145–164. Adams, K.D., and Wesnousky, S.G., 1998, Shoreline processes and the age of the Lake Lahontan highstand in the Jessup Embay- ment, Nevada: Geological Society of America Bulletin, v. 110, no. 10, p. 1318–1332. 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