24 A. G. LEONARD 



are alluvial deposits spread over the surface by the Red River and 

 its tributaries. This fine, thinly laminated silt commonly has a 

 thickness of 30 to 50 feet, though in places it is considerably less. 



Where rivers entered the lake the sediment carried by them 

 accumulated to form delta deposits. Three such deltas are found 

 in North Dakota, the largest, formed by the Cheyenne River, cover- 

 ing an area of about 800 square miles. The sand of this delta has 

 been heaped by the wind into dunes, and a large tract of the delta 

 is covered by these sand hills. 



Recent deposits: alluvium. — Alluvium is found along practically 

 all the streams of the state, being deposited by the rivers over their 

 valley bottoms in time of flood, and in some of the larger valleys 

 it has accumulated to a considerable depth. It is composed of sand, 

 clay, and gravel, the upper 2 or 3 feet being commonly clay. The 

 belt of alluvium along the Missouri River averages 2 to 3 miles in 

 width. As shown by borings, the silt deposited by this river reaches 

 in places a thickness of nearly 100 feet. Some of it may have been 

 deposited during the Pleistocene, but much of this alluvium of the 

 Missouri and other rivers belongs to the Recent epoch. 



Quaternary history.— In conclusion the course of events during 

 the Quaternary period may be briefly described. At the close of 

 the Tertiary the warmth of a temperate climate gave way to the 

 rigors of an arctic cold. North Dakota was several times invaded 

 by an ice sheet and many of the surface features as we find them 

 today are the result of these invasions, particularly of the most 

 recent one. The ice of the earlier invasion crossed the broad and 

 deep valley of the Missouri and extended from 40 to 60 miles beyond 

 that river. The deposits of this older glacier are in most places 

 thin and appear to have undergone much erosion. This drift west 

 of the Missouri may never have been very thick, except locally, 

 where it forms moraines, and much of the finer material of the till 

 has been swept away by the streams, leaving behind the gravel 

 and bowlders. 



This ice invasion produced important changes in the preglacial 

 drainage of the region. The Missouri Valley and the lower valleys 

 of the Yellowstone and Little Missouri rivers were blocked with 



