The natural drainage basins of North America east of the Rocky Mountains are readily described. The Mississippi River and its branches drain nearly all the region lying between the Appalachian chain and the Rocky Mountains and south of the Dominion of Canada and of the Great Lakes. All the southern tributaries to the Great Lakes are insignificant, the river partings on the south being reached in a very short distance. The drainage of the rather limited basin of the Great Lakes is northeastward through the St. Lawrence River, leaving nearly all of the Dominion of Canada east of the Rocky Mountains to pour its surplus waters northward into Hudson Bay and the Arctic Ocean. With the exception of the St. Lawrence River, these are essentially permanent systems of drainage. To understand the extent to which the ice of the Glacial period modified these systems, we must first get before our minds a picture of the country before the accumulation of ice began. _Preglacial Erosion._ Reference has already been made to the elevated condition of the northern and central parts of North America at the beginning of the Glacial period. The direct proof of this preglacial elevation is largely derived from the fiords and great lake basins of the continent. The word "fiord" is descriptive of the deep and narrow inlets of the sea specially characteristic of the coasts of Norway, Denmark. Iceland, and British Columbia. Usually also fiords are connected with valleys extending still farther inland, and occupied by streams. Fiords are probably due in great part to river erosion when the shores stood at considerably higher level than now. Slowly, during the course of ages, the streams wore out for themselves immense gorges, and were assisted, perhaps, to some extent by the glaciers which naturally came into existence during the higher continental elevation. The present condition of fiords, occupied as they usually are by great depths of sea-water, would be accounted for by recent subsidence of the land. In short, fiords seem essentially to be submerged river gorges, partially silted up near their mouths, or perhaps partially closed by terminal moraines. It is not alone in northwestern Europe and British Columbia that fiords are found, but they characterize as well the eastern coast of America north of Maine, while even farther south, both on the Atlantic and on the Pacific coast, some extensive examples exist, whose course has been revealed only to the sounding-line of the Government survey. The most remarkable of the submerged fiords in the middle Atlantic region of the United States is the continuation of the trough of Hudson River beyond New York Bay. As long ago as 1844 the work of the United States Coast Survey showed that there was a submarine continuation of this valley, extending through the comparatively shallow waters eighty miles or more seaward from Sandy Hook. [Illustration: Fig. 50.--Map showing old channel and mouth of the Hudson (dewberry).]