GLACIAL PERIOD, or ICE AGE. Over nearly all of the North American continent north of the 40th parallel and over a vast tract of the continent of Europe, due to the work of moving ice sheets or glaciers, rock surfaces have been ground and polished, great• boulders have been carried and deposited long distances from the ledges whence they came, and the topography has been given characteristically rounded. outlines. Since the marks of the ice chisel are plainly visible on hard rocks, and even on easily weathered rocks that have been protected by a thin layer of soil, it is evident that the ice finished its work recently.
Effects of At the opening of the Glacial Period most of the land surface over which the ice advanced was covered by a deep soil grading through partly decayed rock into solid rock. Undoubtedly the ice did not level off the general surface of the country as much as has been supposed, but it wiped off the soil and partly decayed rock and dumped it into the valleys, rounded the outlines of hills, broadened north and south valleys and pushed before it or carried along a mass of detritus which formed, whenever the ice stopped its ad vance, a terminal moraine. It is possible that clay and boulders were laid down in a thin sheet under the ice in places at least, forming what is known as boulder clay or till and near the southern edge of the ice sheet, producing oval hills of clay and boulders known as drum lins. Other deposits were formed along the edge of the ice, from material worked over by water and known as stratified drift. These de posits include irregular hills of sand, gravel and boulders, called kames, and long winding ridges of the same material called eskers. These latter are supposed to represent the filled channels of subglacial rivers. Irregular depressions known as kettle holes occur in a glaciated region where isolated masses of ice were buried as the ice sheet retreated. It is in fact in the retreat of the ice front that topog raphy was most modified, the terminal moraines, at each pause in the retreat, dammed river valleys, while the valleys were filled sometimes to a depth of hundreds of feet with detritus. Between the morainic dams in front and the ice in the rear, great lakes were formed, one of these, Lake Agassiz in Minnesota, the Dakotas and Manitoba being 700 miles long from north to south. At the same time, the Great Lakes stood at a much higher level than now. Their outlet through the Saint Law rence River was still blocked with ice, and they drained by various channels in part through the Chicago River to the Mississippi, in part by other outlets. At a later stage they drained through the Mohawk Valley into the Hudson River, and thence to the Atlantic Ocean. Their history has been very complex, and is traced largely in the beaches which they left. Since these beaches are not now horizon tal it is known that changes of level have oc curred since glacial times. As the glacial retreat was recent, streams have not had time to cut down valleys and so a glaciated region is a region of lakes.
Cause and Duration of the Glacial Period. — Though several theories of the cause of the Glacial Period have been proposed, no one has received general acceptance. The existence of Glacial periods in past geologic ages is well es tablished. Some writers hold that north ern North America and probably Scandinavia were much elevated at the close of Tertiary time and that this elevation of the land caused so heavy a snowfall that snow lay on the ground all the year round, and glaciers started. Another hypothesis, that of Croll, is that owing to variations in the eccentricity of the earth's orbit around the sun the hemisphere having winter when the earth was farthest from the sun would for a period have protracted winters, and during this period great masses of ice might accumulate. Whatever the cause, the ice sheets formed and advanced. In North America three centres of glaciation are gen erally recognized, the Cordilleran along the Rocky Mountains in British Columbia, whence the ice flowed eastward possibly 1,000 miles or more; the Keewatin, near Hudson Bay, whence the ice advanced southwest, south and southeast, reaching as far south as Iowa; and the Laurentide, north of the Saint Lawrence River and in Labrador, whence the ice advanced over eastern Canada, New England and the Central States as far west as the Mississippi River. The retreat of these ice sheets was ac companied or preceded by changes of level, until at the close of the Ice Age, during the so called Champlain stage, or its equivalent, the Columbian, the ocean covered what is now dry land in the vicinity of Saint Lawrence River and Lake Champlain, and the climate was milder than now. The ice did not advance nor retreat steadily. Some geologists recognize in the Mississippi Valley four or five advances and corresponding retreats, and speak of these as epochs or stages. The time since the close of the Ice Age has been variously estimated; average estimates being around 20,000 years. There is good evidence for believing that as much time elapsed between some of the ad vances of the ice. Hence it is sometimes said that we may be living to-day in an Inter-glacial Period. It is certain that man was in Europe in what is known as the Chelean Epoch, or early Pleistocene. He may have been in America at the same time, but no certain evi dence of his presence has been found. See COLUMBIAN. FORMATION; DILUVIUM ; CHAM PLAIN STAGE; DRIFT; DRUMLIN: GLACIER; TILL.
Bibliography.— Agassiz, 'Systeme Glaciale' (1847); Croll, 'Climate and Time' (1885); Dawson, (Canadian Ice Age' (1894) ; Bonney,