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 evidence 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 arêtes,
and g in
o en forward into trough-like valleys with
over-steepened basal walls. These forms were as
typical of strong glacial erosion as any that I have 1 ~~ ,
seen in the Alps. It was a surprise that features of
this kind could be distinctly recognized at dis- Fig. 55.—Cirques in the Kalkagar-tau.
tances of 3o or 4o miles, yet we were persuaded
that their identification was safe. We had a similar experience when looking from the Alabuga Valley at the Kalkagar-tau, some 3o miles to the south. Several of the cirques there noted are shown in figs. S4 and 55 as sketched through a field glass. All of these cirques are continued downward by narrow, steep-pitching normal valleys, 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 forms, li
while as a matter of fact they are as a rule \~ k
singularly sharp and serrate. Either form alone might be explained without recourse to glacial erosion, but the combination of the two forms, sharp peaks and open val-
leys, is believed to find explanation only
Fg. 56.—Cirque near Sutto-bulak Pass, Kungei Ala-tau.
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