Page:Popular Science Monthly Volume 49.djvu/378

360 The continuation of the Hudson River Valley has been traced by detailed hydrographic surveys to the edge of the steep continental slope at a distance of about one hundred and five miles from Sandy Hook. Its outermost twenty-five miles are a submarine fiord three miles wide and from 900 to 2,250 feet in vertical depth measured from the crest of its banks, which with the adjoining flat area decline from three hundred to six hundred feet below the present sea level. The deepest sounding in this fiord is 2,844 feet. An unfinished survey by soundings off the mouth of Delaware Bay finds a similar valley submerged nearly twelve hundred feet, but not yet traced to the margin of the continental plateau. Again, the United States Coast Survey and British Admiralty charts, as Spencer states, record submerged fiord outlets from the Gulf of Maine, the Gulf of St. Lawrence, and Hudson Bay, respectively 2,664 feet, 3,666 feet, and 2,040 feet below sea level. The bed of the old Laurentian River, as the preglacial St. Lawrence is named by Spencer, from the outer boundary of the Fishing Banks to the mouth of the Saguenay, a distance of more than eight hundred miles, is reached by soundings 1,878 to 1,104 feet in depth. Advancing inland, the sublime Saguenay fiord along an extent of about fifty miles ranges from three hundred to eight hundred and forty feet in depth below the sea level, while in some places its bordering cliffs, one to one and a half miles apart, rise abruptly fifteen hundred feet above the water.

On the Pacific coast of the United States Prof. Joseph Le Conte has shown that the islands south of Santa Barbara and Los Angeles, now separated from the mainland and from each other by channels twenty to thirty miles wide and six hundred to one thousand feet deep, were still a part of the mainland during the late Pliocene and early Quaternary periods. In northern California Prof. George Davidson, of the Coast Survey, reports three submarine valleys about twenty-five, twelve, and six miles south of Cape Mendocino, sinking respectively to 2,400, 3,120, and 2,700 feet below the sea level, where they cross the hundred-fathom line of the marginal plateau. If the land there were to rise one thousand feet, these valleys would be fiords, with sides towering high above the water, but still descending beneath it to profound depths. Le Conte has correlated the great epeirogenic uplifts of North America, known by these deeply submerged valleys on both the eastern and western coasts, with the latest time of orogenic disturbance by faulting and upheaval of the Sierra Nevada and Coast Range in California during the closing stage of the Tertiary and the early part of the Quaternary era, culminating in the Glacial period. In the Mississippi basin, from the evidence of river currents much stronger than now, transporting Archæan