76 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
a northeasterly course. The third capture, west of Crook Mountain, 
was effected by a subsequent stream probably formed by the coales- 
cence of several streams, which by monoclinal shifting had become 
annular with reference to Crook Mountain and the Whitewood Peak 
laccolith. The order of the steps in the development of this portion 
of Whitewood Creek has not been clearly determined; but the facts 
are probably these: lower Whitewood Creek, formerly radial but 
later becoming annular, diverted the waters of Spiegel’s Gap Creek; 
the latter in similar fashion secured the Sandy Creek drainage, 
while Sandy Creek, with the probable assistance of subsequent drain- 
age, developed with reference to Whitewood Peak, succeeded in cap- 
turing the main stream of the region. 
The acceleration of the tilting in later Pleistocene to recent times 
has permitted the incision of the present gorges to the depth of over 
two hundred feet below the former valley floors and has caused the 
superposition of Bear Butte and Whitewood Creeks upon the anti- 
cline east of Crook Mountain. The gravels have been partially dis- 
sected and removed and the reversal of drainage through Spiegel’s 
Gap has been accomplished by recent capture. The main drainage 
features, however, have been but slightly altered. 
THE BIGHORN DISTRICT. 
Tur Gravets. The eastern flank of the Bighorn Mountains 
(Figure 5 and Plate 4), is 
o cloaked with gravels similar 
ET. in some of their aspects to 
Lag those of the Black Hills. 
a These deposits frequently rise 
on the backs of the upturned, 
crescentic masses of Car- 
boniferous limestone toward 
WYOMING rounded notches in the top 
of the ridge. Some parts of 
their slopes are rounded and 
are suggestive of flat, conical 
form; but in the main they 
make a fairly uniform cover 
of no great thickness, which descends from the lower flanks of the range 
toward the plains and conceals the bevelled edges of the upturned Per- 
Y.N. 
+o 
a & 3 
hs 
Ce Sheridan 
Fie. 5. Index map of the Bighorn district. 
