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. . more equable climate, with milder winters and cooler summers, and possibly increased precipitation. Two Pleistocene lakes formed by slope failures in the canyon of Zion National Park were investigated by Hevly (1979). Pleistocene lake Coalpits sediments revealed an abundance of molluscan taxa, including Succinea avara, Stagnicola sp., Pisidium sp., Helisoma sp., and Gyraulus parvus, as well as a Sphaerium clam shell, Azolla sp. (mosquito fern), and a Potomogeton-like pondweed, trace invertebrate tracks, and a camel track. Nearby Trail Canyon lake also contained fragments of molluscan Planorbella tenuis {snail) shells, an unidentified fish and bison vertebrae. These flndings suggest that these lake environments were rather persistent during the late Pleistocene. Holocene Climatic Change Evidence from bog cores and fossils from high elevation glacial cirques provided a transitional record of climatic change during late Pleistocene and Holocene time at 3,600 m elevation in the San Juan Mountains (Andrews et al. 1975). These studies revealed that ice and snow retreated from cirques between 11,000 and 9,000 YBP, and suggest an upslope retreat of treeline at that time. Cores from the recently dewatered lake Emma revealed that one of the largest glaciers in the southern Rocky Mountains had melted away by 15,000 YBP, with timberline ca. 650 m lower than at present and temperatures ca. 50 C cooler (Carrara et al. 1984). Warmer temperatures than present predominated from 9,500 to 3,000 YBP, after which slightly cooler conditions have prevailed. Holocene precipitation patterns apparently shifted from more uniform precipitation to the present strongly bimodal pattern (Chapter 3). Regional aridity increased during the past 10,000 years, with a period of maximum desiccation between 8,000 and 6,000 years ago and somewhat wetter conditions since that time. These climatic changes and the presence of man in the Southwest led to the extinction of the Pleistocene megafauna (Martin and Klein 1984). Climatic changes are also believed responsible for changes in human occupation patterns in the Four Corners area (Euler et al. 1979; Schwartz 1980). Several deeply-incised canyons in Zion National Park were dammed by slope failures through Holocene time, resulting in a number of small lakes. Hevly (1979) examined the paleoecology of five lakes between 1,280 and 1,920 m elevation in Zion National Park and concluded that the slope failures occurred during periods of higher ambient moisture. The lakes that formed in these canyons were shallow, alkaline, ephemeral environments with aquatic and emergent vegetation (sedges, Typha, Myriophyllum). Four kilometer-long Sentinal lake was created by a large slope . . . . . . . . 13 .