PHYSIOGRAPHIC OBSERVATIONS. 135
It is thus seen that, whatever these Mai Valley escarpments are, and however they were produced, their formation took place during the time of the deposition of the new moraines observed. It is also clear, since they support the bottoms of the old broad-trough valleys and are cut by the new, narrower troughs, that their formation must have taken place between the establishment of the old and of the new trough floors. This places the large, old, worn-down moraines as contemporary with the former broad troughs and the new, fresh, narrower moraines as contemporary with the narrower or present trough bottoms.
The Trans-Alai range is largely carved into cirques. We had an excellent opportunity to study those low down on the northern flank of the range. They may be divided into two classes—the very large and the small cirques. These cirques empty either in groups into a trunk trough or open directly into the Alai Valley. The large cirques emptying directly into the Alai Valley come down on a normal slope to the plain, breaking through the escarpments in which their sections form double curves similar to the sections of the twice-troughed valleys. In and in front of them lie piles of moraine with fresh topography.
Fig. 98.—Section across Kizil-Art Valley at Bor Daba, looking north.
The bottoms of small cirques opening directly into the Alai Valley lie high up on the escarpments, and the moraines contained by them are worn down to smooth, low masses. There is no double curve in their cross-section. The whole depression with its moraine is, in cases, much dissected.
The absence of the fresh moraines in the small cirques, and the absence of the secondary depression through the escarpments, is good evidence that these small cirques were carved largely during the epoch or epochs predating the escarpments. From this we may reason that only the large cirques of those in question were, to any great extent, glacially active during the epoch following the formation of the escarpment. It seems probable that this difference was because their upper slopes begin several thousand feet above those of the small cirques, and thus accumulated ice when the line of perpetual snow was above the smaller ones. A careful study of cirques at the critical levels might determine the perpetual snow-line of the later glacial epoch.
Of existing glaciers we had, from a short distance, a good view of three in the respective valleys back of the long moraines described. Each appeared to lie on a moraine. Directly in front of the ice this moraine floor was cut by a small rounded