244 THE ICE AGE IN NORTH AMERICA. 



feet above the Missouri River, extending in a southwest direc- 

 tion from Yankton, Dakota, to the glacial border, a distance 

 of about forty miles, transported bowlders of considerable 

 size are abundant, and among them are numerous specimens 

 of the so-called Sioux Falls quartzite, whose nearest outcrop 

 is about forty miles to the northeast. 



All along upon the eastern side of the river in Dakota 

 the glacial accumulations are on an enormous scale, and the 

 transported bowlders without number; and on crossing the 

 river at Fort Yates, about fifty miles south of Bismarck, 

 granitic bowlders, in numerous instances from three to five 

 feet in diameter, are found resting continuously over a belt 

 of the highlands from 500 to 600 feet above the river, and 

 extending about forty miles to the west, where they suddenly 

 cease. There is granite in the Black Hills, 200 miles to the 

 west, and from that source some pebbles have been brought 

 down the Cheyenne River, which rises in that region. But 

 with this exception, there are no granitic bowlders over the 

 area between the Black Hills and this glacial border just 

 mentioned. The source, therefore, of these bowlders on the 

 west side of the Missouri River, extending from Bismarck 

 to the Nebraska line, must lie somewhere to the northeast. 

 Many of them might well enough have come from the vicin- 

 ity of Lake Superior, a distance of 400 or 500 miles, though 

 possibly some of them originated in more limited outcrops 

 of granite ,in northern Minnesota. 



In British America, the transportation was outward from 

 the Laurentian axis in every direction. From this axis bowl- 

 ders in immense quantities were carried from 600 to 700 

 miles westward and left on the flanks, of the Rocky Mount- 

 ains, from 2,000 to 3,000 feet above their source. 



In Dr. George M. Dawson's report upon the extension of 

 the Missouri coteau into the central region of North Amer- 

 ica, he estimated that nearly ninety-eight per cent of this 

 great accumulation between the Missouri and Saskatchewan 

 Rivers was from the Laurentian axis, some hundreds of miles 

 to the east; and that, upon the fringe beyond the coteau, 



