MANSFIELD: POST-PLEISTOCENE DRAINAGE. ae 
mian to Cretaceous strata, except where they have been exposed by 
later erosion. While these gravels cannot be traced along definite 
Stream courses, as can those of the Black Hills, itis probable that they 
represent a former piedmont slope, formed by the union of contiguous, 
flat, alluvial cones, such as those described by Davis (b, p. 346). They 
have been deeply trenched by the present streams and numerous sec- 
tions are thereby exhibited in which the unconformity of the gravels 
is clearly shown. The interstream uplands are smooth and regular 
and have a marked, but gentle slope of about five degrees in a direc- 
tion slightly north of east. The gravels consist of rounded and sub- 
angular fragments of limestone, sandstone and chert, white quartz, 
Cambrian quartzite, and breccia, such as might easily have come from 
the strata now exposed along the top and flanks and in the canyons 
of the front rampart of the range. Although granite is abundantly 
exposed in the interior of the mountains at the present time and form 
Steep walls for Big Goose Canyon only two miles from its mouth, 
granite pebbles in the gravels are rare or wanting, except that along 
the course of Big Goose Creek, stretching for perhaps one or two miles 
eastward from the mouth of the canyon, there are large, rounded 
granite boulders lying at the level of the gravel deposits, which are 
Similar in many respects to those now found in the bed of the creek 
and were probably deposited by it when flowing at a higher level. The 
pebbles of the gravel have an average diameter of from one to three 
inches; but there are occasional boulders five inches to two feet in 
diameter. Although there are abundant evidences of glaciation in 
the interior of the range, ten to twenty miles back from the front 
rampart, such as moraines, scattered boulders, kettles, ponds and 
marshes, there is no definite evidence that the pebbles of the gravels 
in question are of glacial origin. Any scorings or polished surfaces 
that they may once have had, have been removed by weathering and 
only their subangular form is suggestive of glacial deposition. This, 
however, is not conclusive, since torrent pebbles, if not carried far, 
would not have time to become entirely rounded. The gravels were 
not found in contact with Tertiary deposits; but they overlie uncon- 
formably strata as high in the stratigraphic column as the Laramie. 
From their position and character, somewhat analogous to the like 
features of the Black Hills gravels, they may be tentatively assigned 
to the Pleistocene. 
LITERATURE. The literature of the Bighorn region is not extensive. 
The earlier surveys of the western states and territories gave these 
mountains hardly more than a passing notice. G. H. Eldridge con- 
