Effects of freezing water.—The irresistible force exerted by the expansion of water in freezing, amounting to about 9 per cent of its bulk, is a powerful factor in widening and deepening fissures and cracks of rocks; not uncommonly, whole masses of rock are rent into fragments by this agency, which is one of the most common causes of “rock falls” on the brink of precipices. By the freezing process cracks and crevices are enlarged, and the surfaces exposed to weathering are still farther increased; and the rock fragments or soil particles are loosened and rendered more liable to be removed from the original site, whether by gravity, wind or water.
Glaciers.—Ice in the form of the glaciers that descend from mountain chains ([see figure 1]), and of the moving ice sheets that have covered large portions of North America and Europe in past ages and now cover Greenland and the South Polar continent, exerts a most potent action in abrading and grinding even the hardest rocks; not so much by the direct friction of the moving ice itself, as by the cutting, scoring, grinding and crushing action which the stones imbedded in the ice, or carried and shoved by it, exert upon the rocky channels in which the ice stream moves, as well as upon each other. The product of this grinding process is largely very fine (hence “glacier flour”), so that it remains suspended in the water of the glacier-streams until their velocity is permanently checked when reaching a plain or lake. This suspended stone-flour imparts to the glacier streams their distinctive character of “white rivers,” as contradistinguished from the clear, dark “green rivers” that have their origin outside of glaciated areas. This difference can be readily observed in traveling along any of the glacier-bearing mountain chains of the world, and is frequently expressed in the names of the streams.
Fig. 1.—Zermatt Glacier(Agassiz).
The physical analysis of mud from the foot of Muir glacier,[5] Alaska, at its sea front, made by Professor Loughridge, shows the prevalent fineness of the materials brought down by the glacier waters.
| PHYSICAL COMPOSITION OF GLACIER MUD. | |||
|---|---|---|---|
| Material. | Diameter. | Per Cent. | |
| Clay | ? | 16.57 | 70.31 |
| Fine silt | .0023 — .016 mm. | 53.74 | |
| Fine silt | .016 to .025 mm. | 4.38 | |
| Medium silt | .025 to .036 mm. | 7.06 | |
| Coarse silt | .036 to .047 mm. | 5.91 | |
| Coarse silt | .047 to .072 mm. | 3.76 | |
| Fine sand | .072 to .12 mm. | 1.14 | |
| Medium sand | .12 to .16 mm. | 1.56 | |
| Total | 94.12 | ||
It will be noted that over 70 per cent of this mud consists of extremely fine, wholly impalpable materials; but little of which is true clay.
The fineness of the glacier flour renders it peculiarly suitable for the rapid conversion into soil, and such soils are usually excellent and remarkably durable. The great and lasting fertility of the soils of southern Sweden is traced directly to this mode of origin, and doubtless the great American ice sheet of glacier times is similarly concerned in the high quality of the soil of our “north central” states, from the Ohio to the Great Lakes and the Missouri.
The accumulations of rocks and debris of all sizes in the “moraines” or detrital deposits of glaciers and ice-sheets form another class of glacier-made lands which cover extensive and important agricultural areas (drift areas), both in the old and new worlds. Such lands are undulating or slightly hilly, and the soil usually contains imbedded in it stones of a great variety of kinds and sizes, partly angular, partly rounded and polished by friction. Of course the frequent and violent changes of temperature occurring on the surface of a glacier, aid materially in reducing the rocks carried by it to the condition in which we find the material of the moraines; which commonly form lateral or cross ridges in valleys formerly occupied by glaciers.
Action of flowing water.—The action of flowing water is doubtless at this time the most potent mechanical agency of soil formation. From the sculpturing of the original simple forms in which geological agencies left the earth’s surface into the complex ones of modern mountain chains, to the formation of valleys, plains, and basins out of the materials so carried away, its effects are prodigious. The torrents and streams in carrying silt, sand, gravel and bowlders, according to velocity and volume, do not merely displace these materials; the rock fragments of all sizes not only score and abrade the bed of the rill or stream, but by their mutual attrition produce more or less of fine powder similar to that formed by glacier action; usually more mixed in its ingredients than the former, because derived from a wider range of drainage surface. In the glacier stream itself, it is easy to trace the gradual transition from the sharp stone fragments lying in the water as it issues from the terminal ice cave at the lower end of the glacier, to the rounded shingle found a few miles below.