726 ALLEN DAVID HOLE 
glaciers; in all cases the valleys to which they are tributary were 
glaciated. As to mode of formation, the hanging valleys in this 
quadrangle may be divided into two classes, viz.: (1) those due 
primarily to glacial deposition; (2) those due primarily to glacial 
erosion. 
The valley of Prospect Creek is an example of class (1). The 
lower course of this stream was covered by glacial ice moving down 
the valley of the San Miguel River to which it is tributary, and the 
morainal deposits left by that glacier across the course of Prospect 
Creek have been eroded only in part by the stream, leaving the 
bottom of its valley just outside the moraine still about 350 feet 
above the level of the San Miguel River. Deertrail basin is an 
example of class (2). The bottom of this valley is about 1,500 feet 
above the level of the valley of the San Miguel River to which it is 
tributary. There is no means of determining just how much of the 
valley of the San Miguel was lowered by glacial erosion at this 
point, but from an examination of other tributaries near, it does not 
seem probable that it could have been more than a very small part 
of the 1,500 feet mentioned above. That the San Miguel valley 
was much less flat-bottomed in pre-glacial time than now hardly 
admits of question; the present steepness of the valley wall at the 
point where the stream’ from Deertrail basin enters must therefore 
be due primarily to lateral erosion by glacial ice. As has been 
pointed out by Russell,’ this widening of a valley at the bottom is 
entirely sufficient to produce the phenomena of hanging valleys in 
the case of tributaries with a steep gradient. The very small size 
of Deertrail basin, however, together with the fact that the lower 
end of the basin is approximately at the same level as was the 
surface of the ice in the valley of the San Miguel River, indicates 
that the conditions of glacial erosion primarily responsible for 
making this basin a hanging valley are those stated by Russell for 
mountain-side glaciers,” viz., a gully or other depression occupied 
by a small glacier whose downward limit of erosion was a distance 
above the bottom of the main valley equal to the thickness of the 
ice in the main valley less the thickness of ice in the tributary. 
* Bulletin of the Geological Society of America, XVI, 80. 
2 [bid. i 
