Geology of Lincoln Co., S. D. — Bendrat. 91 
From other smaller fragments a large crystal of a plagio- 
clase may be mentioned, belonging apparently to a basic, por- 
phyritic rock, the axis c of the crystal measuring i^ inch, axis 
b 11/2 inch, while angle bc=83° 30'. 
The gray granites show in some instances reddish felsitic 
bands; the gneisses exhibit in some few cases a flexed and 
bent structure, as though they had undergone dynamic lateral 
pressure. This may also be said of some talcose and chloritic 
schists. 
Although the presence of these bowlders renders the tilling 
of certain parts of the region extremely cumbersome, they are 
in so far cf economic importance that they enable the new- 
comer, whose means are limited, to replace the more expensive 
bricks by these erratics, in foundations and many structures, 
e: Channels and Terraces of the Quaternary. 
The melting of the "Dakota"-ice-lobe and its retreat from 
the region under discussion is supposed to have gone on pari 
passu with a gradual submergence of the land, caused by the 
general downward movement of the surface, inaugurating the 
Champlain epoch. There is assumed to have existed during 
the time of the invasion of the region by the ice a subglacial 
drainage system, when rills and furrows were dug and fed by 
glacial waters which entered along the inner slope of the mo- 
raine as well as through fissures and crevasses in the very body 
of the ice sheet, and which became better developed when the 
ice began to melt faster. The most prominent of these glacial 
channels, us those now occupied by Xine Mile creek and the 
Beaver creeks, have already been mentioned on a previous page. 
There are ;■ number of others within the limits of the region, 
which are now deserted and which are so intimately connected 
with the members of the moraine that they readily betray their 
glacial origin. They are as a rule not very deep and not ex- 
ceeding one mile in their widest portions, while their outlines 
can be traced with greater or less certainty for longer and 
shorter distances. More favorable conditions for their devel- 
opment must have prevailed in the southern part of the area 
than in the north, as they are found more frequently on the 
inner western slope of the "Canton ridge" than along the inner 
southern slope of the northern ridge. Only the most prominent 
are indicated in the map. giving a slight idea of the extension 
