These black piles are the necks or lava plugs of extinct volcanoes. They rise above the level of the plain because, being harder than the surrounding rocks, they have resisted erosion. In some cases these necks or plugs have been converted by shrinkage, on cooling, into beautiful columns, somewhat of the type of the basaltic columns of the Giant's Causeway. It would be difficult to count the number of volcanic necks that can be seen near the edge of the mesa. One's attention is at once attracted to some dozen of these piles, which are especially striking on account of their great size, and ominous black color, but the number is by no means limited to this dozen. There are hundreds of them.
[Fig. 19] gives some idea of a part of the view from the edge of the mesa, and [Fig. 20] the appearance of two of these volcanic necks.
But besides high volcanic mountains such as the San Francisco Mountains and Mt. Taylor, there are, in different parts of the United States, to be found fragments of huge craters from which, in the geological past, immense quantities of lava have escaped. In some instances these craters are but fragments of huge craters, that, like the crater of Mt. Somma, in Vesuvius, have been nearly completely blown away by some unrecorded explosion during the far past.
Fig. 19. Panorama from the Mesa at the Edge of Mt. Taylor
From U. S. Geological Survey
A crater of this type, known as Ice Springs Crater, is situated in the desert valley west of the Wahsatch Mountains, some 125 miles south of Salt Lake City, Utah. This crater is especially interesting from the fact that it occupies a position on a plain that was formed by the deposition of sediment in an immense lake that covered this part of the United States very long before man lived on the earth. We are alluding to Lake Bonneville, a lake that existed in a geological time known as the Glacial Epoch. This lake occupied the territory now filled by the Great Salt Lake of Utah, but towards the close of the Glacial Epoch it was immensely larger than it is now. This can be shown not only by the presence of shore lines, that are clearly marked on the sides of the surrounding mountains, but also by the ancient lake beaches, and deltas, that are common in the district, so that instead of there being the comparatively limited area of Great Salt Lake as marked on the maps of to-day there was a lake that had an area of 19,750 square miles, that covered an area on which at least 200,000 people dwell.
