By
George B. Barbour.
In my recent article on Loess* it was noted that so far no satisfactory explanation has been offered to account for the vertical cleavage which is such a prominent characteristic of the formation. The theories of von Richthofen and Bailly Willis were referred to briefly.
A much more convincing explanation was recently suggested to the writer by Dr. C. P. Berkey, geologist of the Third Asiatic Expedition, on his return from Mongolia. As far as can be ascertained, this particular point of view has not been emphasized by other students of the problem and Dr. Berkey has been so kind as to permit it to be offered here for the first time. He points out, however, that the explanation is based primarily upon observation of conditions at present ruling in parts of the Gobi and upon certain considerations regarding climatic variations in recent times established by researches in Mongolia, rather than upon any extended study of the loess formation itself, which is not extensively developed in the areas explored by the Expedition.
The principles involved are three -(1) continued abundant growth of steppe vegetation, especially coarse grass and bent; (2) contemporaneous and steady supply of fine dust, which was carried into the region by the winds and settled down to the ground, where it was protected by the vegetation from further disturbance and became part of the permanent soil mantle; and (3) subsequent destruction of all traces of such pre-existing vegetation under climatic and ground conditions that led to almost complete oxidation of the organic matter. The principal factor in such destruction is apparently oft-repeated moistening followed by drying and access of fresh air, just as happens when rain waters percolate through the loose ground above the permanent water-level in a region of moderate but frequent rains. This explanation suggested by Dr. Berkey tallies with observed fact in climatically analogous regions to-day. The dust which partially buries the standing blades of growing grass, consolidates round them and tends to preserves the structure lines of the vegetation; it thus possesses from the outset the vertical lines of weakness that lead to the characteristic cleavage afterwards.
With regard to the question of the removal of the evidence by ground conditions especially destructive to organic remains, it might be pointed out that the fossils habitually found in the loess are those capable of resisting such attack from groundwater in the vadose zone, while all more delicate and unstable structures are wiped out; the Struthiolithus shell is frequently found in the loess, whereas not a single example of the bones of the bird itself is known to science.
This explanation in the main reestablishes the picture of conditions given by von Richthofen and revives his vegetation theory so criticized by Kingsmill, Ward and Willis, but with modifications, which, as far as can be ascertained, do not appear to have been put forward in exactly this form by any other student of the problem.
unless otherwise indicated