MANSFIELD: POST-PLEISTOCENE DRAINAGE. 85 
strata the retreat of the cuesta-like wall, by monoclinal shifting, has 
been able all along its length very nearly to keep pace with the ten- 
dency to develop obsequent drainage. The limit of shifting will be 
reached when grade is attained by the subsequent streams; and from 
that time on the retreat of the wall will be slower, because it will then 
depend only upon the slow process of weathering and the chance 
undercutting of the stream. The obsequents, on the other hand, 
will continue to work vigorously, so that their influence upon the 
drainage and topography will become more and more apparent until 
that time when all the activities of the region decline toward old age. 
A Resequent Rill. On the western wall of the Red Canyon wet 
weather drainage flowing down the uncovered slope of the purple 
limestone, has gashed it near the bottom and the gully thus produced 
is growing headward by sapping. Already the incision has reached 
the water table and a permanent rill fed by springs has been estab- 
lished. The direction in which this rill now flows is identical with 
that which a consequent stream would have. The rill, however, can- 
not be termed consequent because during the removal of superior 
strata the surface on which it flows has been produced by the retreat 
of an obsequent slope and the present direction of flow has been, as it 
were, resumed after an interval in which the opposite direction ob- 
tained. It belongs therefore to the class of resequent streams, of 
which there are other examples in Pennsylvania, the Jura Mountains, 
and elsewhere. The radial streams on Crook Mountain and other 
domes in the Black Hills are probably also members of this class; for 
the slopes on which they run are certainly not the original surfaces 
of the domes and, in the development of the present surfaces by the 
removal of superior strata, several generations of obsequent slopes 
may have successively retreated along the dip of the beds, each retreat 
being followed by the development of resequent drainage until the 
present status was reached. 
SUMMARY. 
In the Bighorn Mountains, as in the Black Hills, the Pleistocene 
period seems to have been marked by extensive denudation and the 
‘accumulation of gravel deposits. Since the deposition of the latter, 
Big Goose Creek and other streams have been permitted to entrench 
themselves deeply below their former valley floors. That this incision 
is due to uplift or broad up-warping rather than to climatic oscillation 
