Fine examples of such superimposed streams which are now entirely out of harmony with the structure of regions through which they flow are the Susquehanna, Delaware, and Hudson. Thus the Susquehanna cuts across a whole succession of Appalachian ridges while, in accordance with the same explanation, the Delaware cuts through the Kittatiny range or ridge at the famous Delaware Water Gap. The ridges are explained as follows: while the great master streams were cutting deep trenches or channels in hard and soft rock alike, numerous side streams (tributaries) came into existence and naturally mostly developed along belts of weak, easily eroded rock parallel to geologic (folded) structure. Thus the Appalachian valleys have been, and are being, formed, while the ridges represent the more resistant rock formations which have more effectually stood out against erosion. The lower Hudson River flows at a considerable angle across folded formations above the Highlands, after which it passes though a deep gorge which it has cut into the hard granite and other rocks of the Highlands. The simple explanation is that the Hudson had its course determined upon the surface of the upraised Cretaceous peneplain, and that it has been able to keep that course in spite of discordant structure and character of the underlying rocks. In a similar manner we may readily account for the passage of the Connecticut River through a great gap in the Holyoke ridge or range of hard lava in western Massachusetts. Before leaving this part of our discussion we shall briefly present some evidence showing that the New York-New England-St. Lawrence region at least must have been considerably higher shortly before the Ice Age (Quaternary period). An old channel of the Hudson River has been traced about 100 miles eastward beyond the present mouth of the river and it forms a distinct trench under the shallow sea in the continental shelf. Even in the Hudson Valley, many miles above New York City, the bedrock bottom of the river lies hundreds of feet (near West Point, 800 feet) below sea level. Obviously this submerged channel must have been cut when the land in the general vicinity of New York City was fully 1,000 feet higher than at present. That the land thus stood higher late in the Tertiary and possibly early in the Quaternary periods is proved as follows: (1) because most of Tertiary time must have been needed for the river to erode such a deep valley after the initial uplift of the peneplain about the beginning of the period; and (2) because glacial deposits of Quaternary age filled the former channel to a considerable depth. The valleys of the coast of Maine, and the submerged lower St. Lawrence Valley (Gulf of St. Lawrence), in a similar way lead us to conclude that the region farther north was also notably higher just before the Ice Age.