In North and also in South America that age was characterized by one notable event, which impressed its presence by lasting memorials on the surface of the continent. This was the formation of a series of enormous glaciers, covering the soil of nearly half the temperate zones with a mass of ice thousands of feet in thickness. The period of its presence is called the Great Ice Age or the Glacial Epoch. Beyond the immediate limits of the ice it may not have been a season of extreme cold, for glaciers form more rapidly when the temperature is not much below the freezing point. Nor was it continuous. The ice sheet receded once, if not twice, causing an “interglacial” epoch, when the climate was comparatively mild. After this interim it seems to have advanced again with renewed might, and to have extended its crystalline walls down to about the fortieth parallel of latitude, touching the Atlantic near Boston and New York harbors, and stretching nearly across the continent in an irregular line, generally a little north of the Ohio and a little south of the Missouri rivers. Enormous ice masses covered the Pacific Slope as far south as the mouth of the Columbia river, and extended over 1200 miles along the coast, submerging the whole of Queen Charlotte and Vancouver islands and the neighboring coast of British Columbia, which at that time were depressed about two hundred feet below the present level. The ice also covered for four hundred miles or more the plateau or Great Basin between the Rocky Mountains and the Coast Range, rising in some places in a solid mass five or six thousand feet above the soil.[7] The melting of the second glacial inroad began at the east, and on the Pacific coast has not yet ceased. Its margin across the continent is still distinctly defined by a long line of débris piled up in “moraines,” and by a fringe of gravel and sand called the “overwash,” carried from these by the mighty floods which accompanied the great thaw. This period of melting is the “Post-glacial Era.” It was accompanied by extensive changes in the land-levels and in temperature. In the glacial and early post-glacial periods, the northern regions of the continent and the bottom of the Northern Atlantic were considerably above their present levels; but in the late post-glacial or “Champlain” period the land had sunk so much that at Lake Champlain it was five hundred feet lower than now, and at New York Harbor ten feet lower. The St. Lawrence river was then an arm of the sea, Lake Champlain was a deep bay, and the mouth of the Delaware river was where the city of Trenton now stands, the river itself being a wide inlet.[8] The climate, which in the early post-glacial period had been so cold that the reindeer enjoyed an agreeable home as far south as Kentucky, changed to such mildness that two species of elephants, the giant sloth and the peccary, found congenial pasturage in the Upper Ohio and Delaware Valleys.[9]