As the water flowed away from the neighborhood of the glacier it formed large streams which carried great quantities of glacial material in suspension. These it worked over, assorted, and laid down in deposits which differed in no way from those described in preceding sections which treat of transported clays. § 21, 22, 23, 24, 29.
Origin of 32. Sometimes the water spread out in broad, thin sheets which, having little velocity, deposited a fine silty mate rial over broad areas, carrying the finer clays still further away. Later when the water had disappeared and the surface had become dry this silty deposit, whether worked over by the winds or not, became loess. Such deposits may accumulate until they attain great thickness. As the loess question is a matter of controversy and the way in which it was formed is still unsettled, it may be well, without attempting an extended discussion, to say here that loess is a siliceous silty clay. Silt is a finely granular material, usually composed in large part of unde composed rock particles, or in other words of rock flour, but often con taining coarser clay granules as well. Loess, then, is largely made up of rock flour, fine quartz sand and coarse clay. Such a deposit could only be made by an agent which could assort its material, for we know of no way in which so large a mass of uniform in texture could be produced directly, hence deposits of loess must have been produced through the action of wind or water or of both. Wind and water, however, can only transport, assort and deposit the loess. They cannot make the silty grains, hence the richer the transported material is in silt the more probable will be the formation of loess deposits. Silt may be composed of particles of those minerals which resist decomposition most effectively and consequently more or less of it may be formed when rocks break down under the action of ground water and other agents of disintegra tion, or it may be produced by abrasion of wind, water. or ice-borne
fragments against exposed rock surfaces, or its grains may be formed by the agglutination of particles of clay. The following propositions may, I think, be accepted as established.
The granular material which we call loess can be found in most earthy deposits.
Such material is more abundant in the deposits of dry than in those of humid regions. Such materials are more abundant in the deposits of the later glaciers than in those of the earlier glaciers, rivers, or in masses produced by decomposition of rocks, but are found in these also.
If such materials, no matter how they originated, are properly assorted and the different grades laid down in separate masses, loess-like deposits will be formed.
' Water and wind working separately or together are the only agents that can effect this assortment and decomposition.
Water would tend to form broad sheets of nearly uniform thickness while winds would deposit their load in heaps.
Conditions are more favorable to the assortment and deposition of loess in dry than in ordinarily humid regions, because there the surface oftener becomes thoroughly dry and consequently the winds are there more effective and because the rains come in heavy local showers which pick up large quantities of the dry and loosened dust and carry it into streams which when they leave their narrow beds and spread out upon the plains assort and deposit it again.
The large quantities of water and sluggish currents that attended the melting of the great glaciers were peculiarly favorable to the formation of deposits of loess.
Such granular deposits would whenever they become dry be peculiarly liable to be worked over, transported and redeposited by winds.
The loess problem so far as it concerns the history of individual de posits will not be solved until we come to understand the geographic, physiographic and climatic conditions which prevailed during each sub-stage of the glacial and post-glacial periods.