THE OVERLAlfD iROtJM— COUiTClL BLtrFFS TO OGDEN. 



n 



Iler^ and elsewhere in central and eastern Nebraska large quantities 

 of grain are raised. Much of it, especially the corn, is fed to live 

 stock. Animals raised on the western ranges are shipped here for 

 fattening before they are sent to the market. 



In the river bluffs along Platte Valley southeast of Columbus are 

 the westernmost deposits made "by the continental glaciers. East of 

 a north-south line passing a httle east of Columbus the su23erficial 

 deposits consist of loess and of glacial till containing bowlders and 

 fragments of rock brouglit from the north by the glaciers during one 

 of their first southward advances in the Great Ice Age, some features 

 of which are described below by W. C. iVlden.^ Tliose deposits make 

 relatively high rolhng plains. West of this line the surface of the 



^ Many of the physical features of east- 

 ern Nebraska were produced by sheets of 

 ice that invaded the region during and 

 after the earlier stages of the Great Ice 

 Age. The deposit best exposed, in the 

 street cuts and river bluffs in and near 

 Omaha and along the line of the Union 

 Pacific to the west, is a dustlike clay or 

 loess. Beneath this lies the glacial drift. 



Another feature is the great Missouri 

 River, which swings majestically back 

 and forth across its broad valley bottom 

 as it gathers in the waters of the Great 

 Plains on their way to the sea. In late 

 Tertiary time, before the advent of the 

 earliest continental ice sheet, Missouri 

 River as now known was not in existence. 

 The Dakotas were drained to Hudson 

 Bay, and northeastern Nebraska was 

 probably drained southeastward across 

 Iowa. Platte River may have joined 

 Grand River in Missouri. The bedrock 

 east and west of the present lines of bluffs 

 lies relatively low in the Omaha region, 

 so that before the coming of the glaciers 

 there was probably only a valley of 

 moderate size with low slopes instead of 

 bluffs. 



The close of Tertiary time and the be- 

 ginning of Quaternary time was marked 

 in the northern part of the United States 

 by the formation and spreading of vast 

 sheets of ice similar to the great ice cap 

 that now envelops all but the marginal 



parts of Greenland. 



mild 



equable climate of the Tertiary period 



or \iolent 



ecessarily 





lowerini 



the average annual temperature a few 



degrees — so that a large part of the precipi- 

 tation came in the form of snow, which 

 was not all melted away in the summer. 

 As this snow remained from season to 

 season a vast amount finallv accumulated 

 and formed great glaciers. There were 

 three main centers of accumulation and 

 dispersion of this glacial ice, one on the 

 Labrador Peninsula, a second west of 

 Hudson Bay in the district of Keewatin, 

 and a thii^d in the mountains of western 

 Canada. (See fig. 4, p. 22.) 



At the opening of the glacial epoch the 

 great Keewatin glacier spread southward 

 and covered large parts of the Dakotas, 

 Minnesota, and Iowa and extended thence 

 into eastern Nebraska, where it was 

 probably several hundred feet thick. 

 The dark-blue clay containing pebbles 

 and small bowlders which is exposed 

 near the base of the river bhiffs in South 

 Omaha and near Florence, several miles 

 north of Omaha, is a part of the deposit 



this 



It is 



known as pre-Kansan, sub-Aftonian, or 

 Nebraskan glacial till. As the front of 



the great ice sheet in\aded the Dakotas 

 and Nebraska the eastward-flowing 

 streams were blocked and their water was 

 turned southward. This wat-er must 

 have formed a stream somewhere west of 

 Omaha. 



This first stage of glaclation was brought 

 to a close by the melting of the ice in a 

 warmer interglacial time or stage — the 

 Aftonian. 



of the region swept great quantities of 

 sand and gravel down their valleys. 

 Remnants of these sand and gravel 



During. this stage the streams 



