CIRQUES IN THE TIAN SHAN. 
91 
V. 
Forms of the kind thus characterized are not to be accounted for by the theory 
of normal erosion, and are not found where normal erosion has acted alone. They 
may, however, be accounted for by means of reasonable assumptions as to glacial 
action, and they occur, so far as is known, only in mountains that are otherwise 
proved to have been glaciated. Their occurrence may therefore be taken in evi- 
dence of the verity of the reasonable assumptions by which they are explained; all 
the more so because the process of glacial erosion is not open to direct observation, 
and because there are many different opinions as to its rate, method, and amount. 
For these very reasons it seems warrantable to determine the process rather by the 
forms that follow from it than by the little that can be seen of it in actual operation. 
The different glacial ranges of the Tian Shan that we saw either close at hand 
or in the distance afforded repeated examples of 
nearly every feature above named as characteristic 
of glacial action. Mention has already been made 
of the snowy range seen to the south of the Fer- 
gana basin, where our field glasses showed numer- 
ous cirques crowned with sharp peaks and aretes, 
and opening forward into trough-like valleys with 
over-steepened basal walls. These forms were as 
typical of strong glacial erosion as any that I have 
seen in the Alps. It was a surprise that features of 
this kind could be distinctly recognized at dis- 
tances of 30 or 40 miles, }'et we were persuaded 
that their identification was safe. We had a similar experience when looking 
from the Alabuga Valley at the Kalkagar-tau, some 30 miles to the south. Several 
of the cirques there noted are shown in figs. 54 and 55 as sketched through a 
field glass. All of these cirques are continued downward by narrow, steep-pitching 
normal valle>-s, with respect to which the cirques would have to be explained as the 
product of an earlier cycle of erosion if 
they were not admitted to be of glacial ori- 
gin. But if the open cirques were regarded 
as of normal origin, the mountain summits 
above them ought to be rounded fonns, 
while as a matter of fact they are as a rule 
singularh' sharp and serrate. Either fonn 
alone might be explained without recourse 
to glacial erosion, but the combination of 
the two fonns, sharp peaks and open val- 
leys, is believed to find explanation only 
by the special process of glacial erosion. 
While we were crossing the low ranges south of Issik Kul on July 16, a 
number of cirques and troughs were seen high up in the Terskei Ala-tau to the 
south and southeast. Some of the cirques opened on the walls of the larger troughs 
in true hanging-valley fashion. One of the troughs showed with remarkable 
Fig. 55. — Cirques in the Kalkagar-tau. 
Fg. 56. — Cirque near Sutto-bulak Pass, Kungei Ala-tau. 
