We have still to explain the existing topography or relief of a large and important part of eastern North America, including the whole of the Appalachian Mountains, Allegheny Plateau, Piedmont Plateau, New York, New England, and the Canadian region to the north. As a starting point in this discussion we should recall the fact that, after the great Appalachian Mountain Revolution toward the close of the Paleozoic era, the predominant geologic process which affected the region under consideration was erosion throughout the succeeding Mesozoic era. By about the close of the Mesozoic (Cretaceous period) the whole region, with some local exceptions, has been worn down to a comparatively smooth plain (peneplain) not far above sea level. Local exceptions were mainly in the New York and New England region as, for example, some of the higher parts of the Adirondack and White Mountains, Mount Monadnock in southern New Hampshire, and Mount Greylock in western Massachusetts. These and other masses rose rather conspicuously above the general level of the great plain of erosion commonly called the "Cretaceous peneplain" because it is believed to have been well developed by the close of that period. The uplift of the vast Cretaceous peneplain about the beginning of the Cenozoic era (Tertiary period) was an event of prime importance in the recent geological history of eastern North America because it was literally the initial step in bringing about nearly all of the existing major relief features of the Appalachian-New York-New England-St. Lawrence region. The amount of uplift (unaccompanied by folding) of the peneplain was commonly from a few hundred to a few thousand feet with the greatest amount in general along the main trend of the Appalachians. The fact should be emphasized that nearly all the principal topographic features of the great upraised region have been produced by dissection (erosion) of the uplifted peneplain surface. Thus nearly all the valleys, small and large, including those of the St. Lawrence, Hudson, Mohawk, Connecticut, and Susquehanna, have been carved out by streams since the uplift of the great peneplain.