The continental shelf is 50 to 100 miles wide in this sector. Toward Cape Hatteras the coastal plain widens as the shelf narrows. The combined features are called the "emerged and submerged coastal plain." The gradient of the continental slope ranges from 1:8 to 1:15 and the base of the slope with one exception is at 1150 ± 100 fathoms. The shelf break is at about 50 fathoms on all profiles. On profiles W-12, W-13, W-14, W-15, W-18, and Figure 1 of Plate 4 there is a second break at 75-100 fathoms.
The break between the continental slope and the upper continental rise is abrupt at some places and occupies a distance of 5 to 10 miles in other places (Fig. 16). In each case the gradient of the next lower 30- to 50-mile segment is 1:100.
All profiles from Georges Bank to Cape Hatteras, a span of more than 500 miles, show both an upper and a lower continental rise (Profiles W-13, W-19, and Fig. 15). The uniformity in the continental slope gradient carries over into the continental rise. Both the upper continental rise and the lower continental rise are divided into three segments. The width, gradient, and depths of each of the slope segments are remarkably similar. Representative values based on profiles W-13 to W-19 and Figure 15 are shown in Table 1.
Figure 15.—Continental margin provinces: Type profile off northeastern United States.
Profile plotted from PDR records. This profile is representative of the sector from Georges Bank to Cape Hatteras.
| Depth | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Segment | Upper edge | Lower edge | Gradient | Width | ||||||||
| Upper continental rise | ||||||||||||
| 1 | 1150 | ± | 150 | 1450 | ± | 200 | 1:100 | ± | 20 | 30 | ± | 5 |
| 2 | 1450 | ± | 200 | 1650 | ± | 150 | 1:275 | ± | 25 | 40 | ± | 15 |
| 3 | 1650 | ± | 150 | 2150 | ± | 200 | 1:90 | ± | 30 | 30 | ± | 15 |
| Lower continental rise | ||||||||||||
| 1 | 2150 | ± | 200 | 2350 | ± | 100 | 1:250 | ± | 50 | 40 | ± | 10 |
| 2 | 2350 | ± | 100 | 2350 | ± | 100 | 1:1400 | ± | 1000 | 50 | ± | 20 |
| 3 | 2350 | ± | 100 | 2725 | ± | 100 | 1:150 | ± | 30 | 60 | ± | 20 |
The upper continental rise and the lower continental rise are essentially terrace or shelflike features. Each has a relatively steep (1:50-1:200) outer face (segment 3) and a relatively gentle (1:250-1:2000) shelflike surface (segment 2). In each case a slope of intermediate gradient (1:80-1:300) (segment 1) connects the upper shelflike surface with the next higher face. In the case of the upper continental rise the next higher face is the continental slope. Other smaller-scale terracelike features may eventually be correlated along the strike when more data are available. The local relief exceeds 20 fathoms in the deeper parts of segment 3 of the lower continental rise. A range of hills extends for a few hundred miles along the base of the continental rise as indicated on the physiographic province chart (Pl. 20). These hills, known as the lower continental rise hills, are 30-100 fathoms high and each is 1 to 3 miles wide. An echogram (Pl. 3, fig. 4) shows three continental rise hills. The only other part of the continental rise where relief of more than 20 fathoms is generally encountered is in segment 1 of the upper continental rise. The irregularity in this case is probably related to the extensions of numerous continental-slope canyons onto the continental rise. Relief of 5-10 fathoms is almost universal in segments 1 and 2 of the upper continental rise. The echogram reproduced in Figure 1 of Plate 3 shows typical minor-relief features of the upper continental rise. An oblique crossing of a submarine canyon on the upper continental rise is shown in Figure 2 of Plate 3. The smooth topography typical of most of the remainder of the continental rise is well illustrated by the echogram shown in Figure 3 of Plate 3.