6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ledges of the trough; and the rapids increase in fall as the stream 
passes certain breaks or steps in the trough bed, thus departing 
very clearly in several ways from the well-graded flow of a stream in 
a normally mature valley. The lower end of the southeasternmost 
spur of Mt. Elbert is all hacked and torn where it was wrapped 
around by the Lake creek glacier about a mile southwest from the 
village of Twin Lakes; the spur, therefore, shows a fine piece of well 
advanced but not well finished glacial erosion in the rough ledges 
and the high cliffs that here truncate the well graded slopes of its 
upper or superglacial parts. 
All these features have an expression distinctly unlike those pro- 
duced by normal processes. The hanging lateral valley or trough of 
Crystal lake gulch differs most conspicuously from a normal lateral 
valley in its lack of accordance with the main valley. The broad 
cirque head of the lateral valley is in strong constrast to the more 
delicate valley heads of the neighboring normally eroded, non- 
glaciated mountains. Just in what manner glacial erosion works 
to produce these systematically correlated abnormal features is not 
yet fully made out, but that they represent strong glacial modifica- 
tions of normal preglacial forms is beyond question. As to the 
measure of such erosion, the depth of the main trough below the side 
trough may be taken as a fair minimum; for it is extremely im- 
probable that the mouth of the hanging lateral can be today no 
deeper carved than the corresponding point in the preglacial side 
valley. A peculiar feature is seen in the ravining of the sides of the 
main trough by several small streams from gulches between the trun- 
cated spurs to a greater depth than by the stream of Crystal lake 
gulch. The latter has done very little work in cutting a notch in the 
lip of its hanging mouth; the former in several cases have ravines, of 
small size to be sure yet of relatively normal quality, eroded to depths 
that must be hundreds of feet lower than the upper limit of glacial 
scouring on the trough sides. Interglacial work is probably con- 
cerned here; but why it was not successful in ravining the mouth of 
the hanging valley is not clear. These questions I leave to Pro- 
fessor Westgate for solution. 
A matter that still needs distinct statement in problems of this sort 
is that, in ascribing to glacial erosion the strong modification of pre- 
glacial forms above described, it is not to be implied that the processes 
of glacial erosion are fully understood. Sapping by temperature 
changes is believed to take place in the bergschrund around the head 
of a cirque; but the depth of the cirque beneath the névé surface 
