Loess.---Loess, a fine grained porous deposit of material accum ulated under the influence of winds blowing out from an ice-sheet across its marginal moraines, is widespread in South Russia and Turkestan. It gives rise to different soil types, the chernozyom being carried farther into moist conditions on loess because the porosity of loess drains off the superfluous water. Under arid conditions grey semi-desert and desert soils are formed on loess. When moister and milder conditions of climate supervened during the "Atlantic" phase of climate after the great retreat of the glaciers, forests sprang up in the regions affected, though appar ently less on loess than elsewhere. Under forest the humus was removed by displacement of sesqui-oxides and colloidal clay from upper to deeper horizons, the resulting soil being termed degraded or licked (leached) chernozyom. The map shows this belt stretch ing in a south-west to north-east direction in European Russia and extending into Asiatic Russia as far as the Altai mountains, with a few patches further to the east as far as the trans-Baikal region. A tongue of it extends as far north as Perm and another tongue extends northwards on the left bank of the Vyatka. The oak forest is characteristic of the northern belt of degraded chernoz yom in European Russia and there are three stages of develop ment, the belt where the forest spread at the expense of the steppe and was continuous until man interfered, the belt of island like clumps of oak near the rivers, and the steppe belt, where forest is found in the ravines only and the plateau is left to steppe vegetation. This latter belt has had interesting historic relations; the Slays took to the forested ravines and left the plateaux to the nomad grazers, and to the present day village settlements run continuously along the ravines. The proximity to river water was an attraction, though ravine streams often disap pear in summer except after heavy showers.
On plateau land enclosed in the angle between two rivers, up land oak occurs (e.g., the Voronezh province oak used by Peter the Great for ship-building). Associated with the plateau oak forest are ash, lime, maple and elm. The fertility of the rich black earth is such that many years of continuous cropping hardly affects it and it is this zone which made Russia a famous grain exporting region even in the time of Greek and Roman dominion. The islands and ravine strips of oak cease south of lat. 48° N., and of the line from the Stalingrad (Tsaritsyn) bend of the Volga to the Urals south of lat. 52° N. Patches of oak occur near the left bank of the Volga from the bend tc 52° N. Thus the Pontic steppe was left free from forest and was a highway for nomad invaders. For types of agriculture, difficulties and history of the region see Ukraine and Black Earth Region (Central). In Asiatic Russia the railway runs through the chernozyom belt where Russian settlement has been intense since the latter part of the 19th century. The more easterly extensions of the steppe black earth lie in the zone of recurrent droughts and terrible famines, and under stress of famine conditions the ancient tradition of persistent wheat sowing on the three-field system is giving way to cultivation of maize and potato and to many-field systems; irrigation schemes are also being developed.