422 
STRATIGRAPHICAL GEOLOGY 
is not known whether it should really be regarded as a part of the 
White River formation, or considered a remnantal bed that might 
belong to some horizon of the Pierre, Fox Hills, Laramie, Union, 
or Miocene sections. 
In distribution, thickness and composition this gravel bed is quite 
irregular. Its localization usually in old swales separated on the 
same level by beds of Fuller’s earth, clays, and sands, indicates 
original stream deposition. In thickness the bed varies from a 
few inches to a dozen or more feet. Locally there is passage up¬ 
wards into soft, fine-grained sandstones. The pebbles compris¬ 
ing the conglomerate are mainly the size of a hen’s egg, but vary 
from half an inch to several inches in diameter. Boulders 12 to 
20 inches in diameter are not of infrequent occurrence. 
This gravel bed, as a rule, is found in remnantal patches occu¬ 
pying relatively large areas, and it is disposed in such manner as 
to make it almost impossible to prove how it got there. The 
region over which it occurs is deeply eroded, the span of denuda¬ 
tion extending from the Late Tertic period to the Present time. 
During Pleistocene times the clays, silts and shales associated near 
the surface were often extensively reworked. 
Regarding the exact position of this gravel bed in the regional 
geological column the recent evidences appear to indicate that it 
is an integral part of the Chadron formation. The first locality 
in which it was definitely and positively discovered in undisturbed 
position at this horizon was ten miles northwest of Crawford, 
Nebraska, in the “Bad Lands” on the south side of Big Cotton¬ 
wood Creek. All over this general region, the Chadron forma- 
ation lies unconformably upon the “Rusty” member of the Pierre 
shales. The juncture is nearly always well marked. The lower 
few feet of the Chadron section here, are a blue-gray silt mixture, 
which when disturbed remains a long time in suspension in water. 
Over large areas, this lower section of the Chadron is traversed 
by numerous chalcedony seams, nodules, and concretionary chunks. 
The chalcedony is evidently a deposition formed after the beds 
were in position. 
It is in this same lower phase of the Chadron, immediately above 
the Pierre shale, from a few inches to ten feet, that this coarse, 
silicious “gravel” occurs. The pebbles do not occur in concentrat¬ 
ed pockets as a rule, but are scattered through the silt. When one 
