Some of the proofs of the former presence of the great ice sheet are as follows: (1) polished and striated rock surfaces which are precisely like those produced by existing glaciers, and which could not possibly have been produced by any other agency; (2) glacial bowlders or "erratics" which are often somewhat rounded and scratched, and which have often been transported many miles from their parent rock ledges (Plate 20); (3) true glacial moraines, especially terminal moraines, like that which extends the full length of Long Island and marks the southernmost limit of the great ice sheet; and (4) the generally widespread distribution over most of the glaciated area of heterogeneous glacial débris, both unstratified and stratified, which is clearly transported material and typically rests upon the bedrock by sharp contact. The best known existing great ice sheets are those of Greenland and Antarctica, especially the former, which covers about 500,000 square miles. This glacier is so large and deep that only an occasional high rocky mountain projects above its surface, and the ice is known to be slowly moving outward in all directions from the interior to the margins of Greenland. Along the margins, where melting is more rapid, some land is exposed, and often the ice flows out into the ocean where it breaks off to form large icebergs. [Illustration: Fig. 47.--Map of North America showing the area buried under ice during the Great Ice Age of the Quaternary period; the three great glacial centers; and the extent of mountain glaciers in the west. (After U. S. Geological Survey.)] The accompanying map shows the area of nearly 4,000,000 square miles of North America covered by ice at the time of maximum glaciation, and also the three great centers of accumulation and dispersal of the ice. The directions of flow from these centers have been determined by the study of the directions of many thousands of glacial scratches on rock ledges. The Labradorean (or Laurentide) glacier spread out 1,600 miles to the south to Long Island and near the mouth of the Ohio River. The vast Keewatin glacier sent a great lobe of ice nearly as far south, that is into northern Missouri. "One of the most marvelous features of the ice dispersion was the great extension of the Keewatin sheet from a low flat center westward and southward over what is now a semiarid plain, rising in the direction in which the ice moved, while the mountain glaciers on the west (Cordilleran region), where now known, pushed eastward but little beyond the foot-hills." (Chamberlin and Salisbury.)