During my first boyish exploring trip in the Rocky Mountains I was impressed with the stupendous changes which the upper slope of these mountains had undergone. In places were immense embankments and wild deltas² of débris that plainly had come from elsewhere. In other places the rough edges of the ca?ons and ridges³ had been trimmed and polished; their cliffs and projections⁴ were gone and their surfaces had been swept clean of all loose material. Later, I tried vainly to account for some ca?on walls being trimmed and polished at the bottom while their upper parts were jagged. In most ca?ons the height of the polishings above the bottom was equal on both walls, with the upper edge of the polish even or level for the entire length of the ca?on. In one ca?on, in both floor and walls, were deep lateral⁵ scratches in the rocks.[Pg 248]

One day I found some polished boulders⁶ perched like driftwood on the top of a polished rock dome⁷; they were porphyry, while the dome was flawless granite⁸. They plainly had come from somewhere else. How they managed to be where they were was too much for me. Mountain floods were terrible but not wild enough in their fiercest rushes to do this. Upon a mountainside across a gorge⁹ about two miles distant, and a thousand feet above the perched boulders on the dome, I found a porphyry outcrop; but this situation only added to my confusion. I did not then know of the glacial period, or the actions of glaciers¹⁰. It was a delightful¹² revelation when John Muir told me of these wonders.

Much of the earth's surface, together with most mountain-ranges, have gone through a glacial period or periods. There is extensive and

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