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In the subsequent retreat of such a glacier, imperfect terminal moraines might be formed higher up, where the water is not deep enough to form icebergs. It is probable, too, that since the melting of the great "mer de glace" and the formation of the Lake, the level of the water has gone down considerably, by the deepening of the Truckee Canyon outlet by means of erosion.
Efforts to establish the age of the deposit by observations on the yearly growth, would afford little satisfaction, for the obvious reason that conditions governing the growth are dependent, in a measure, on each season's vegetation. Deposit began, of course, after the erosion of the chamber ceased, and therefore represents only a fraction of the age of the cave itself.
The Mesozoic peneplain has been upwarped by later crustal movements and has suffered profound erosion, but the remnants of it which remain on the upland of southern New England and the even summits of the Allegheny ridges suffice to prove that it once existed.
The transition from one to the other of these various conditions was gradual and orderly: first, a nearly simple tableland; then a grand mer de glace shedding its crawling silver currents to the sea, and becoming gradually more wrinkled as unequal erosion roughened its bed, and brought the highest peaks and ridges above the surface; then a land of lakes, an almost continuous sheet of water stretching from the Sierra to the Wahsatch, adorned with innumerable island mountains; then a slow desiccation and decay to present conditions of sage and sand.
Perhaps, he thought wryly, it was a symptom of the gradual erosion of his moral character in this abnormal environment. "I'm getting stale," he confided to Copper as he sat in his office idly turning the pages of the Kardon Journal of Allied Medical Sciences. "There's nothing to do that's interesting." "You could help me," Copper said as she looked up from the pile of cards she was sorting.
In a word, all bowlders, whether angular or rounded, are supposed to owe their origin or separation and shaping to glacial agency. Now, if such be the true view of glacial erosion, evidently its effect in mountain sculpture must be small indeed. Roches moutonnées are recognized by all as the most universal and characteristic sign of a glacial bed.
The masses presented all the appearance and detail of erosion shown by the great mountain mass of the country itself; looking at one of these little models, only a few feet across, and then gazing out upon the great tangle of mountain peaks around us, one could almost imagine that the one was the intentional reproduction of the other, in miniature.
In the hills of Cabo Blanco there are found among the gneiss, angular masses of opaque quartz, slightly translucid on the edges, and varying from grey to deep black. I do not think it constitutes a vein. The waters of the lake* decompose the gneiss by erosion in a very extraordinary manner. It may be drunk without being filtered.
Only the geologist knows the part played by erosion in shaping the earth's surface as we see it. He sees, I repeat, the phantoms of vanished hills and mountains all about us. He sees their shadow forms wherever he looks. He follows out the lines of the flexed or folded strata where they come to the surface, and thus sketches in the air the elevation that has disappeared.
Every scientific agriculturist knows that erosion is one of the chief causes of loss in soil fertility and that in the basins and deltas of streams and rivers there is going to waste enough muck to make all of our land rich. But the cost of getting this fertility back to the soil has thus far proved too great for us to undertake the task of restoration.
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