STRATIGRAPHICAL GEOLOGY 
423 
notes the nature of the Chadron matrix, it seems impossible that 
water which had the power to transport such large stones, would 
not have removed and carried on such fine material with it, or 
certainly better segregated the very coarse from the very fine 
materials. 
A few miles northwest of Chadron, Nebraska, is a group of 
hills, weathering rapidly, where the outcrops of the “gravel” is 
clear-cut and easily seen. Typical Chadron fossils are found in 
position in it. 
The query at once arises what was happening in this area 
through the long Late Cretacic and Eocene times, while the Lara¬ 
mie and its immediately succeeding beds were being deposited to 
great thicknesses at very short distances to the west and north. 
If the “Rusty” layers of the Pierre shales, or the Ainsworth 
formation as it is sometimes called, is a true stratigraphic unit and 
not merely the upper oxidized surface of the eroded and weathered 
Pierre formation, then there existed a condition wherein neither 
erosion or deposition occurred for a long period of years, or else 
that later deposits were superimposed and again removed to a 
nicety exactly down to the “Rusty” Pierre surface, before Oligo- 
cene times set in. Since the Pierre shales are eroded with utmost 
ease, this suggestion seems wholly untenable. However, whatever 
were the events during the Eocene time it appears that at the 
beginning of the Oligocene epoch an interval existed when there 
were excessive flood activities which distributed large volumes of 
gravel and large pebbles over a wide, relatively level country. 
There is definite evidence of at least one localized stream line 
of this period which carried coarse, sharp sands and gravels, and 
which in one area is firmly cemented. The layer shows notable 
cross-bedding. Erosion has largely removed the marks of this 
old channel, so that now only remnants remain. 
Among the coarse pebbles are found occasionally fragments of a 
' peculiar rose quartz. The only locality at present known where 
this quartz occurs in position near enough to have furnished the 
pieces is the area around Custer, among pegmatites in the heart 
of the Black Hills uplift. If the Black Hills did not already have 
a protruding core at the opening of Oligocene times, and if they 
came up after that epoch, as some authors maintain, whence came 
this rose quartz. 
