the 4,000-foot level, and one at an altitude of 4,000 feet is from 800 to 900 feet high, and completely crosses and dams up the ravine down which the glacier formerly

came.

Some have supposed that there are indubitable evidences of former glaciation in the mountain-ranges of southwestern Africa between latitude 30° and 33°, but the evidence is not as unequivocal as we could wish, and we will not pause upon it.

The mountains of Australia, also, some of which rise to a height of more than 7,000 feet, are supposed to have been once covered with glacial ice down to the level of 5,800 feet, but the evidence is at present too scanty to build upon. But in New Zealand the glaciers now clustering about the peaks in the middle of the South Island, culminating in Mount Cook, are but diminutive representatives of their predecessors. This is indicated by extensive moraines in the lower part of the valleys and by the existence of numerous lakes, attributable, like so many in Europe and North America, to the irregular deposition of morainic material by the ancient ice-sheet.*

* See With Axe and Rope in the New Zealand Alps, by G. E. Mannering, 1891.

CHAPTER VII.

DRAINAGE SYSTEMS AND THE GLACIAL PERIOD.

WE will begin the consideration of this part of our subject, also, with the presentation of the salient facts in North America, since that field is simpler than any field in the Old World.

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 Amer

ica 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.

The more accurate surveys conducted from 1880 to

1884 have brought to our knowledge the facts about this submarine valley almost as clearly as those relating to the

FIG. 50.-Map showing old channel and mouth of the Hudson (Newberry).

inland portion of it above New York city. According to Mr. A. Lindenkohl,* this submarine valley began to be noticeable in the soundings ten miles southeast of Sandy Hook. The depth of the water where the channel begins is nineteen fathoms (114 feet). Ten miles out the chan

* Bulletin of the Geological Society of America, vol. i, p. 564; American Journal of Science, June, 1891.

nel has sunk ninety feet below the general depth of the water on the bank, and continues at this depth for twenty miles farther. This narrow channel continues with more. or less variation for a distance of seventy-five miles, where it suddenly enlarges to a width of three miles and to a depth of 200 fathoms, or 1,200 feet, and extends for a distance of twenty-five miles, reaching near that point a depth of 474 fathoms, or 2,844 feet. According to Mr. Lindenkohl, this ravine maintains for half its length "a vertical depth of more than 2,000 feet, measuring from the top of its banks, and the banks have a nearly uniform slope of about 14°. The mouth of the ravine opens out into the deep basin of the central Atlantic.

With little question there is brought to light in these remarkable investigations a channel eroded by the extension of the Hudson River, into the bordering shelf of the Atlantic basin at a time when the elevation of the continent was much greater than now. This is shown to have occurred in late Tertiary or post-Tertiary times by the fact that the strata through which it is worn are the continuation of the Tertiary deposits of New Jersey. The subsidence to its present level has probably been gradual, and, according to Professor Cook, is still continuing at the rate of two feet a century.

Similar submarine channels are found extending out from the present shore-line to the margin of the narrow shelf bordering the deep water of the central Atlantic running from the mouth of the St. Lawrence River, through St. Lawrence Bay, and through Delaware and Chesapeake Bays.* All these submerged fiords on the Atlantic coast were probably formed during a continental elevation which commenced late in the Tertiary period, and reached the amount of from 2,000 to 3,000 feet in the northern part of the continent.

* See Lindenkohl in American Journal of Science, for June, 1891.