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92 streams, diverting them east. Some 10,000 years ago, another tributary, working its way southwest, capturing the southeasterly flowing Platte River near Elm Creek. 28 So, during the Pleistocene, the climate periodically got colder and the ice sheets built up and moved south. Then the climate warmed and the ice sheets melted and retreated back north. As they were retreating, the land got very dry and high winds (paleowinds) deposited thick layers of loess over eastern and southeastern Nebraska. The map below uses the National Hydrography Dataset laid over the glacial till map with the Level III Ecoregions outlined. The National Hydrography Dataset shows all drainages that have been carved into the landscape, many of which are dry drainages. Loess is a highly erodible soil and the areas marked "Loess" in the map show where these deep loess deposits are located. Note that the "Sand Hills" shows only a few drainages. This is because, while sand is very erodible, it is also highly permeable. So rain doesn't run off and erode the Sand Hills landscape, it simply soaks in. During the glaciations, the areas directly under the ice would have had no wildlife. Nothing could have lived under several hundred meters of ice. The areas adjacent to the ice would have been tundra with limited wildlife. The literature says the loess would have been deposited as the glaciers were melting but the deposits would have taken hundreds of years to develop. Wildlife, including fishes and crayfishes, could have lived here during this process. The Sand Hills would have been the last landform to develop, apparently starting some 8,000 years ago, long after the ice had left. They too, would have taken hundreds of years to form. So, crayfish could have been living in the streams in these areas and, from here, could have moved into the glaciated areas as new drainages developed. CRAYFISH AND GLACIERS What possible relevance could this long- winded discussion have to the crayfish of Nebraska? Crayfishes have been with us for a very long time. Fossil crayfishes (the ancestors of our modern crayfishes) have been found that date back to the Triassic (more than 216,000,000 years). 161 These fossil crayfish, while of long extinct species, are clearly related to those we have now. The crayfishes we find in eastern North America are thought to have originated in the area called the Eastern Highlands, an area centered on the Appalachian Mountains extending from Alabama to Pennsylvania. From here they extended their ranges west to the mountains, south into Mexico and north into Canada. By the time the Pleistocene began, streams and lakes throughout eastern North America were well populated with crayfishes. Nebraska drainages as shown by the National Hydrography Dataset overlaying a glacial till map with the Level III Ecoregions outlined.