254 GEOLOGY. 



Often, though not always, this blue clay has intercalated be- 

 tween its layers these thin strata of sand and pebbles. In Saline 

 County where they occur the clay sometimes shades into sand and 

 emerges from it the same way. This clay is a characteristic feature of 

 the earliest deposits of the Quaternary over the greater part of south- 

 ern Nebraska and over a considerable section of north Nebraska. In 

 south Nebraska it occurs in at least three-fourths of the counties. 

 It is brought to light more frequently in boring for water, but oc- 

 casionally it also crops out in railroad cuts, ravines and small canyons. 

 Its thickness ranges all the way from five to sixty feet. Where 

 free from mechanical admixture of sand, it is exceedingly compact 

 and hard. An augur penetrates it with great difficulty, and in such 

 cases it almost bids defiance to a pick. Occasionally it is full of 

 pebbles, many of which lie lengthwise the direction of the glacial 

 path, and, like the underlying rock, are marked by parallel striaea. 

 At other places, instead of pebbles and small boulders, it is inter- 

 mixed with sand in greater or less quantity. In such places it 

 readily permits of the passage of water, but where pure it is imper- 

 vious. In most of these characters it bears a striking resemblance 

 to the English till.* This till Geike shows was first formed be- 

 neath glacial ice (Moraine Profjnde). A body of ice 3,000 feet 

 thick moving forward a few inches or feet in a day would crush 

 and pulverize everything beneath it. This thickness, at least of 

 the glacial mass, can be inferred from the depth of the ice mass in 

 the east, where valleys 5,000 feet deep were filled, as is known by 

 the scorings that crossed them and were made at that height on the 

 bounding mountains. Boulders are also known to have been car- 

 ried across equally elevated mountains. It was nature's mighty 

 millstone to reduce to powder the stony fragments and organic ma- 

 terials beneath it. On the final retreat of the glaciers this-fine, im- 

 palpable mud in part accumulated at the lower end, and in part was 

 carried away by the rushing streams to be deposited in quiet 

 waters. In some such way it became somewhat irregularly laid 

 down over the land. The Erie clays described on the north side of 

 Lake Erie by Sir W. Logan had, according to Newberry, who 

 studied them so thoroughly in Ohio, a similar origin. All such 

 clays, according to these eminent authorities, owe their character, 

 physical and chemical, directly or indirectly to glacial action. In 

 Ohio Newberry, however, regards the Erie clays as a result of the 



*See ' 'The Great Ice Age," by James Geike. 



