Geology of Lincoln Co., S. D. — Bendrat. 8i 
ate, and is very often found to constitute a firm conglomerate, 
tinged a reddish brown by iron oxide, while the sand grains of 
the finest material are very well rounded and of great clearness 
and translucency. Minute grains of garnet and also mica plates 
are often as^'ociated with them. 
A very fine sand, which covers the road leading up the di- 
vide south of Beloit, and which might be met with also in the 
southern part of the region, assuming in rearranging itself at 
both sides of the road a laminar structure, seems to belong to 
a phase of the drift, underlying the till, having been and still 
being washed down by rains from that horizon. This layer, 
which apparently replaces at points the coarser material of the 
drift and which may be considered a transitionary link between 
the sands and gravels and the sandy clays of the till, consists of 
a fine clayey sand, horizontally stratified and of light gray color, 
which nicely contrasts against the bright brick red of oval iron 
accumulations, 2 to 3 inches long, holding in a blackish nucleus 
delicate pappus-like threads of gypsum. These accumulations 
are, however, not confined to this layer, but also occur higher up 
in the till. The upper limit of this deposit is at places very dis- 
tinctly marked, while its base has been observed only in one in- 
stance, viz. : 2j^ miles west of Fairview in the bluffs of the Big 
Sioux, where it is found at a level of about 100-105 feet above 
the level of the river, underlain by the Niobrara chalky series. 
The thickness of this phase of the drift formation amounts at 
this point to nearly 52 feet, while about four miles west of Fair- 
view it is only exposed for 30 to 35 feet. The layer reported 
by Todd from the Big Sioux, 3^4 miles cast of Fairview. con- 
sisting of "fine sand with large rusty concretions" and repre- 
sented in Bull. No. 158 of U. S. G. S., p. 83, seems to belong 
to the same phase. 
The thickness of the sand and gravel deposit as revealed by 
borings, although it varies greatly in diflferent parts of the 
region, would average nearly the same as that reported for the 
clayey satids. viz. : 40 to 50 feet. 
The role, which these sands and gravels play in the water 
supply of the region, has already been referred to. Shallow 
as w^ell as deep wells are furnished by the drift, the water of 
wliich is in some cases charged with iron, in others with alkali, 
but in most cases fairly palatable for man and beast. In the 
