728 ALLEN DAVID HOLE 
basin, be sufficient to account for the lack of topographic adjust- 
ment as seen today. A complete explanation of the present 
difference between these two valleys must, without doubt, include 
both of the modifying conditions (1) and (2), just named. 
In the case of Marshall Creek, only the second of these two 
modifying conditions can apply. This stream enters the San 
Miguel valley from the north at a somewhat steeper grade than does 
Bear Creek from the south, yet there is no abrupt change to a steep 
grade in its lower course such as is characteristic of streams in 
hanging valleys. Comparing again with Bridal Veil basin, the area 
drained by Marshall Creek is considerably less, so that so far as 
wear of channel due to ice alone is concerned it would seem that the 
valley of Marshall Creek would have been lowered by a less amount 
than was the valley of Bridal Veil Creek. The rock exposed in the 
bottoms of the two valleys is in this case the same, so that the only 
other modifying condition which appears to be sufficient to explain 
the difference seen today is that of the pre-glacial topography. 
That is, a gradient in pre-glacial time considerably less steep for 
Marshall Creek than in the case of Bridal Veil Creek or Ingram 
Creek is sufficient to cause the present difference. If this con- 
clusion in the case of Marshall Creek be correct, it would seem 
probable that the modifying influence of pre-glacial topography 
predominated also in the case of Bear Creek, aided in a subordinate 
way by the presence of sedimentary instead of igneous rocks in the 
lower part of its course. 
3. Rounded topographic forms.—Topographic forms rounded in 
outline due to glacial action, in contrast with the angular outlines 
usually found in unglaciated, mountainous areas, occur at many 
points in the quadrangle; they may be grouped as to origin in two 
classes, namely, (1) those due primarily to glacial erosion; and (2) 
those due primarily to glacial deposition. Class (1) includes (a) 
rounded, projecting points or masses of rock in place of which 
roches moutonnées are the type; and (0) valleys having a U-shaped 
cross-section as opposed to the sharper V-shaped section charac- 
teristic of unglaciated mountain valleys of steep grade. Class (2) 
consists of morainal deposits of various kinds, forming low, round- 
topped hills or ridges; sometimes these hills are in the bottoms of 
