In the reconnaissance studies of the Arequipa region, two principal subjects were investigated. One was the enormous deposits of tuff, originating as incandescent flows of Katmaian type, which cover most of the region; and the second was the climatic conditions that prevailed during Pleistocene time, and their effect upon topography. Since a large part of the evidence on climate is found in the tuff deposits the two subjects are closely inter-related, and cannot be wholly separated. Nevertheless it has been thought best to describe the tuffs in one article, immediately preceding this, as representing a certain phase of volcanic activity; and the topographic features and their significance in another. A preliminary reading of the first paper will help in the better understanding of the present paper.

Considerably before the establishment of glaciers on the high mountains north of Arequipa the lower country had been covered by a series of incandescent tuff flows. In these, deep quebradas (gulches) had been cut and a new valley of Chili River had been established before the glaciers became active. This erosion was accomplished by torrential streams, and a pluvial period preceding glaciation is indicated. Next, the gulches were filled with bowldery sediments, which also spread over the intervening pampa surfaces, and finally the sediments were almost completely removed. These later events are regarded as the consequences of the overloading of glacial streams with detritus, followed by still greater flow of glacial melt-waters.

The belief that during the Pleistocene there were great glaciers at work on the high mountains bordering the basin is based chiefly on the presence of huge heaps of bowlders at the bases and on the slopes of the mountains. As the importance of glaciers in this and other Andean regions has been questioned by some observers, the deposits are described in detail and the possibility of their having been formed by other processes is examined. It is concluded that no other process is satisfactory as the principal agency. Other processes doubtless co-operated, but it is believed that glaciers were the chief means by which the materials were gathered up, worked over, and deposited where now found.

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