As the glacier moves forward sand and bowlders frozen into or resting on the ice are carried for long distances. Since it is held rigidly by the ice, the material being transported undergoes no sorting, a marked feature of glacial as compared with river transportation.
Along the sides of a valley next to its rocky banks much material accumulates on the ice. These ridges are known as lateral moraines and when by the junction of two glaciers a moraine occupies a central poiltion 'Wit known as a medial moraine. After the melting of the ice these are 'rarely preserved as distinct ridges. More important is the material accumulated at the front of the glacier, when the rate of melting just balances the rate of advance and each bowlder brought forward is contributed to the growing mass, known as a frontal. or terminal morame. In the case of continental ice sheets such ridges of clay, sand and gravel may be traced for many ,miles. The material is usually unsorted and unstratified. Rounded elliptical hills of unstratified glacial debris with their longer axes in the direction of ice move ment are known as drumlins. They are co mon in southern Wisconsin, central New and the vicinity of Boston. Their cause is not well understood.
Much water, the result of melting, usually flows away from an ice front, tending to pro duce stratifiedlacio-fluvial deposits. A river flowing from the front of a glacier fills the valley bottom for miles with a valley train Of stratified sand and gravel. Spread out in a broad sheet in front of the ice, suchdeposit is called an outwash plain. Large of
ice are frequently buried in outwash plains and melt very slowly, the resulting depressions, often occupied by lakes, being known as kettle holes. Eskers, long, winding ridges of strati fied gravel greatly resembling railroad ettfbank ments, are believed to be deposited by streams flowing in tunnels under the ice. Glacial in de- posits n general are known as drift and are classified as stratified drift, and as till or un stratified bowlder clay.
In regions that have been strongly scoured by ice the preglacial topography is largely ob scured, the original drainage being entirely dis arranged by the complete filling of the old valleys. Such a region shows many features of extreme youth, as lakes and swamps, falls and rapids. Bed rock where exposed may be striated and polished, and many erratic bowl ders, foreign to the country, lie strewn every where, having been brought by the ice from distant regions. • It is known that several times in geologic history glaciers have been more widespread than now. During the Pleistocene most of Canada and a large part of northern United States south to the Ohio and Missouri rivers was cov ered by an ice sheet as was also most of north ern' Europe. During the Permian there was widespread continental glaciation in South Africa, Australia and India reaching into the torrid zone both north and south of the equator. Continental glaciation occurred in the Cambrian period in China, and probably during the Algonkian in Canada. See GLACIER;