Geikie describes loess as typically a "fine-grained, yellowish, calcareous, sandy loam, consisting very largely of minute grains of quartz with some admixture of argillaceous and calcareous matter."[48] It is for the most part a wind-blown deposit. It is widely developed over low-lying regions, but sweeps up to heights of 200 to 300 feet and more above the bottoms of the great river valleys. Again, in many places we find it heaped up under the lee of hills, the exposed windward slopes of which bear no trace of it. Wherever there is loess we are likely to find the remains of steppe plants and animals. The ancient steppe area which generally covers, and probably extends considerably beyond, the loess district, is the region occupied by most of the primitive settlements. Even to-day it is less wooded than the rest of northern Europe. Such steppe regions in the North German plain are the great diluvial river terraces, especially the terraces of the Saale and Elbe and the eastern edge of the Harz Mountains; in South Germany the lower Alpine "Vorland" from Switzerland to lower Austria, the uplands of Suabia and Franconia, the valleys of the Main and Neckar, and much of northern Bohemia. These steppe regions of Germany, northern Austria, and Switzerland extended southeastward in a zone following the Danube, widening out in the great Hungarian plain into the vast steppe region extending eastward from the Black Sea or Pontus. From this Pontic steppe a band of more or less open country extended northward along the Carpathians until it almost or quite joined the open regions of the Elbe and along the Harz. A farther extension of this same band seems to have opened the way from the Harz region through northwest Germany into Belgium and northern France, and very probably into Brittany. We see at once the importance of these long lines of open or thinly forested country to the immigrations and settlement of Neolithic peoples. Periodical floods or other conditions kept open many river valleys, whose importance we shall estimate in a later chapter. All this land, except the uplands of Suabia and Franconia, and some similar areas, was comparatively fertile, the loess areas particularly so, and suited to a primitive agriculture.