Many of the lakes are due to the presence of the terminal moraines, while others fill rock basins.
Russell has described the terraces or gigantic stone steps by which many of the canons rise, and the deep-walled cirques at their heads. About Mount Whitney these features are illustrated on a grand scale. The vast rocky amphitheaters at the heads of the canons are bounded by precipitous walls one thousand to two thousand feet high, while their bottoms are generally occupied by lakes in basins of solid rock.
Lake Mono is at present gradually rising, thus differing from Owen's Lake. The history of these sheets of water has been a checkered one. Periods of high water have alternated with those of desiccation. The last high-water stage seems to have been contemporaneous with the glaciation. Well-defined terraces appear about Mono Lake, indicating the various stages of the water. In this region the opportunities for the study of the glacial phenomena are excellent, for only a small part of it has as yet been investigated.
What is perhaps of more interest than the glaciers is the recent volcanic action so remarkably exemplified about Mono Lake. Following the andesitic flows which took place near the close of the Miocene and in the early Pliocene, there were numerous eruptions of basalt, although some of the basalts of this region may be considerably older. South of Owen's Lake the basalt flows are numerous and extensive, volcanic action in that region being most strongly marked in the Coso Mountains. The most recent eruptions of all have, however, taken place in the vicinity of Mono Lake. Here the surface of the flows is often so fresh and free from soil that it seems as if they had but just cooled. Prom the lake southward for twenty miles there is a line of volcanic cones known as the Mono craters, which are connected with the most recent eruptions and possess great interest. The cones have been built up on extensive obsidian flows, the whole forming a considerable mountainous elevation. The glassy eruptions came first, then the cones were built up of lapilli and scoriaceous material, while the volcanic ash which covers the country for miles around was probably the last to be thrown out. These recent glassy lavas were very viscid and cooled in thick masses instead of thin sheets. This is finely shown in the southernmost of the eruptions, where the glistening, glassy mass which broke into fragments as it cooled rises in almost precipitous crags fifty to seventy-five feet. The bottom of the valley between two different coulees is occupied by a small stream which runs over an older lava. From the surface of the latter, which is covered with soil, spring immense pine trees which must have sprouted since the glass cooled, for no vegetation could have withstood the heat. A
