Fig. 84. - Loess deposits; North China. (Photograph by Willis).

In arid regions the wind often carries the finer parts of the soil to immense distances and deposits them where they are less exposed to the wind, and where there is vegetation enough to hold them. In Central Asia the sun is often darkened for days by these dust-storms, and after they are past, a fine deposit of yellow dust is found over everything. Loess is a deposit formed in this way, and it is found in many lands. One of the largest known accumulations of it is in northern China, where it covers an immense area, to depths of 1000 to 1500 feet. It is not stratified, but cleaves vertically, and thus the ravines and valleys excavated in it have very abrupt sides. Loess also occurs in Europe, and the Pampas of the Argentine Republic are covered with a great thickness of it. The loess of the Mississippi valley, though of rather exceptional character, is yet probably of aeolian origin.