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Show Pacific Ocean across the Sierra Nevada to the Great Basin. With refinements in glacial chronologies and modelling, and including the realization that the Laurentide ice sheet was far more influential than Antevs thought, this hypothesis has withstood testing by computer simulations and field data ( COMAP members, 1988; Kutzbach et al, 1993; Thompson et al., 1993). It is important to keep in mind that Antevs was working at a time when there was no way to correlate between the Bonneville basin and localities in North America or Europe. Antevs thought that the astronomical hypothesis of climate change formulated by Milutin Milankovitch was probably valid, and therefore he was eager to find correlative climatic events across North America and in northern Europe. During the second half of the 20th Century many people have studied paleolakes in the Bonneville basin ( Machette and Scott, 1988; Sack, 1989). Rather than review the sequence of changing interpretations through this period, I will briefly summarize what is now known, and what major questions remain unanswered. Although there are thick and extensive deposits of Miocene lakes in the basin, little is known about the shapes of the early basins and how they evolved. The deposits are poorly exposed and highly faulted. Lake deposits of Pliocene age ( 5- 2 million years B. P.) also are poorly exposed, but in recent years, coring of some of these units has provided information on the size and character of Pliocene lakes. The history of Quaternary lakes in the basin is relatively well known, especially that of the youngest lake cycles, because the stratigraphic and geomorphic records are well preserved and datable. The first large Quaternary lake in the basin reached its highest levels just prior to the eruption of the Lava Creek B ash from the Yellowstone caldera about 660,000 yr B. P. ( Figure 2). Following a long period when lakes remained relatively low, the Pokes Point lake cycle probably Ah occurred between 300,000 and Figure 1. Fluctuations of Lake Bonneville ( L. B.), Lake Lahontan ( L. L.), and Northern Hemisphere ice sheets ( adapted from Antevs, 1948). L. S. = Lake Stansbury, T. L. = Thinolite Lake, L. P. = Lake Provo, D. L. = Dendritic Lake. #* - S° r *<* ^ : 0 A 200 400 500 600 300 age ( to) Figure 2. Schematic diagram of middle and late Pleistocene lake cycles in the Bonneville basin. Modified from Machette 700 200,000 yr B. P., based on studies of amino acids in gastropod shells ( McCoy, 1987). Known deposits of \ the Pokes Point lake cycle are exposed only at Little Valley in the Promontory Range. Deposits of the Little Valley lake cycle ( about 140,000 yr B. P.) are exposed at a number of and Scott ( 1988, Fig. 3), and Oviatt and Miller ( in press, Fig. 2). , ,•*• ,..,., ... 1 * 7 y v & ' localities around the basm, but it is |
