North America
9781465650306
208 pages
Library of Alexandria
Overview
In beginning the study of the physical geography of North America, one of the first facts to claim attention is that the true continental border is in general many miles seaward from the present margin of the land. The boundary of our field of study is defined with considerable accuracy by a line drawn on the bottom of the sea adjacent to the present coast-line of the continent so as to pass through all points where the soundings show a depth of 100 fathoms of water. This 100-fathom contour in the topography of the sea-floor chances to coincide in a general way with the outline of the submerged border of the continent; landward from it the bottom rises with a gentle slope, while seaward the descent is usually steep down to a depth of 2,000 or more fathoms. A gently sloping shelf-like border surrounds the deep central basin of the Gulf of Mexico (Fig. 3). To the west and north of Yucatan and west of Florida the shelf is from 140 to 160 miles broad, with a surface slope towards the centre of the Gulf of less than 6 feet to a mile—a slope so gentle that were the surface of the shelf exposed to view, no eye could distinguish it from a perfect plain. The deepest sounding yet obtained in the central part of the Gulf, approximately midway between Yucatan and Florida, shows a depth of 2,119 fathoms. The remarkable fact is that the slope from the 100-fathom line to the bottom of the central basin of the Gulf is precipitous. In two places on the border of the Yucatan bank a descent of about 8,500 feet occurs within a horizontal distance of 15 or 20 miles. On the east side of the southern extremity of Florida, and again on the eastern shore of Yucatan, the continental shelf is only about 5 miles broad; these are the nearest approaches of the present land to the actual border of the continent to be found on the Atlantic coast. The explanation of these exceptional conditions is that both Florida and Yucatan are portions of the continental shelf which have been raised so as to form low emerged plains. From Cape Hatteras northward to the extremity of the Newfoundland Banks the shelf increases gradually in breadth from about 15 miles in the region of the Carolinas to over 100 miles off the coast of Maine. The outer border of the shelf is an irregular curving line. Opposite the coast of Massachusetts and Maine an extension of the Atlantic basin reaches within 15 or 20 miles of the present margin of the land.
