A wind-borne rock dust (very fine sediments, silt) carried from outwash deposits and moraines and laid down as a thick stratum during periglacial conditions in the steppe country surrounding the ice sheets. Wind erosion was widespread in the periglacial zone that surrounded the large Quaternary ice sheets. Material was picked up by the wind from the large expanses of proglacial deposits at the ice sheet margins. Because of its exceptional fertility, areas of loess were chosen for settlement by early agriculturists. In central and eastern Europe, as well as Asia and North America, there are notable concentrations of sites on loess. It provided good grazing for the animals on which Palaeolithic man fed, was rich in nutrients for plants, and was later settled by Neolithic farmers who found it easy to till with primitive equipment. It is an essentially unconsolidated, unstratified calcareous silt; commonly it is homogeneous, permeable, and buff to gray in color, and contains calcareous concretions and fossils. Loess is important archaeologically as soil erosion in these regions during the Holocene caused substantial redeposition of this silt, often burying (deeply) and preserving archaeological sites. In semiarid regions people such as the Pueblo Indians made houses and fortresslike closed edifices from loess-based adobe.