976 Glacial Drift of the Basin of Iowa. 
in reality does not reach the subjacent rock strata (except at two or 
three points) until Plymouth, in Cerro Gordo county, Iowa, isreached. 
From this point to its confluence with the Cedar, fifty-seven miles 
below, it has eroded its channel down through the drift formation, 
and flows upon the underlying Devonian strata. 
Stretching away from the morainic border, and partially filling the 
valley of the Shellrock, is a train of Valley drift, which extends to 
the confluence of this stream with the Cedar, seventy-two miles 
below. This morainic border lies upon rolling country, and there 
leads away from it, in portions of Cerro Gordo and Worth counties, 
valleys or trough-shaped depressions of notable size, which we 
should naturally expect would have drawn very considerable 
drainage floods from the margin of the ice. The material of 
this formation is usually well rounded, coarse and fine gravel 
and silicious sand, not essentially differing from the Upland 
drift gravel, except that it contains a very much larger percent- 
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age of silicious sand, and does not usually contain any percepti- 
ble admixture of ey The material (except the very coarse 
portion) is everywhere obliquely and E stratified. This 
feature is well illustrated in the following sec 
Fig. 1 represents a section of Valley drift at Bekied: illustrating 
the flow and plunge style of stratification. The material is mostly fine 
