BLOWN SAND 1 25 



times, however, a soil-covered area is depressed beneath the sea 

 or a lake, in such a way that the soil is not washed off, but new 

 deposits are at once laid down upon its surface, and then the soil 

 may be preserved for an indefinite period. Such an old soil, or 

 "dirt bed," may be recognized by its texture and appearance and 

 by the roots, stems, and leaves of land plants, with which it is apt 

 to be filled. Old soils are also preserved in certain cases by lava 

 flows, which have been poured out over them. 



Talus and Breccias. — At the foot of cliffs and mountain slopes, 

 great masses of talus, or angular blocks of all sizes, accumulate, 

 chiefly through the action of frost. These masses form quite 

 steep slopes and show an imperfect division into layers, and they 

 are continually, but for the most part slowly, moving downward, 

 through the action of the same forces that produced them. By the 

 deposition of some cementing material (usually CaC0 3 ) the angu- 

 lar blocks may be consolidated into a solid mass, which is called 

 bi-eccia, and of which the peculiarity is that the fragments compos- 

 ing it are angular, not rounded. 



Loess. — In arid regions the wind often carries the finer parts 

 of the soil to immense distances and deposits it where it is less ex- 

 posed to the wind, and where there is vegetation enough to hold 

 it. 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 accumu- 

 lations 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 is believed to have been laid 

 down in water, under somewhat exceptional conditions. 



Blown Sand. — Wherever a sandy soil occurs unprotected by 

 vegetation, as in deserts or along the seacoast, the wind drifts the 

 sand and piles it up into hills or sand dunes. The dunes are 

 roughly divided into layers, the thickness and inclination of which 



