800 WALLACE W. ATWOOD 
time it was modifying the forms encountered, changing the shape of 
the great canyons, and building new forms. 
Before the first Pleistocene snows fell on the Uinta Mountains, the 
heads of the great canyons may be fairly assumed to have been narrow 
V-shaped notches, reaching in most cases nearly to the crest-line of 
the range. ‘The first ice was formed in these narrow canyon heads, 
and the earliest movement must have been distinctly down canyon. 
As the ice at the heads of the canyons increased in thickness, there 
came to be a notable movement down the sides of the gorge, concen- 
trating the ice in the canyon and causing further movement down 
stream. With the movement of the ice on the side slopes, these 
miniature catchment basins were both widened and lengthened. 
About the margin of the ice fields weathering and ice plucking was 
in progress. Such work has been pointed out by Johnson’ to be 
going on today at the base of the Bergschrund. In this way the 
catchment areas were increased in size and changed into cirque-like 
forms (see Fig. 4). Such work has been described by Penck? as 
follows: 
Glaciers not only exercise a sapping action along their sides, but also at 
their very heads, if they are here overlooked by rock cliffs. There is always a 
marginal crevasse, called in German, Randspalte or Bergschrund, which sepa- 
rates the moving ice from the rocks which overlook it. The material loosened 
here by weathering falls down from the rock walls into this crevasse and arrives 
at the bottom of the névé, where it is pushed forward by the mass grinding the 
bottom of the glacier. By this, not only the formation of screes around the 
glacier is hindered, but also the surrounding cliffs are constantly attacked, for 
the erosive action begins just at their foot and saps them. Glaciers therefore, 
which are formed on slopes in broadly open valley basins, surround themselves 
finally by cliffs, which are pushed backward much as are the cliffs around the 
gathering basin of a torrent. 
Just south of the main crest-line, weathering and the plucking 
work of the ice went on under the most favorable conditions. There 
the strata are essentially horizontal and sufficiently variable in hard- 
ness to favor rapid disintegration. The giving-way of the softer beds 
left the overhanging harder strata exposed. Cliff and talus slopes 
developed, and the ice, working sideward and headward, or in general 
t Journal of Geology, Vol. XII, p. 573. 
2 Journal of Geology, Vol. XIII (1905), p. 15. 
