this tectonic activity produced the basis of scotland's topography: ancient mountains in the north and south of the country, partially eroded by 400 million years of water and ice with a wide fertile valley between them, and a newer, wilder western terrain. with scotland now in the northern temperate zone, it was subjected to numerous glaciations in the neogene and quaternary periods, the ice sheets and their attendant glaciers carving the landscape into a typical postglacial one, overdeepening river valleys into the characteristic u-shape and leaving the upland areas covered with glacial corries and dramatic pyramidal peaks. in lowland areas the ice deposited rich fields of fertile glacial till and eroded the softer material surrounding the extinct volcanoes (particularly the older carboniferous ones), leaving many crags.