The Madeira Abyssal Plain occupies 150,000 square miles. The Azores-Gibraltar Ridge forms its northern boundary, and the abyssal hills its western boundary. Depths near the seaward limit of the plain reach 2970 fathoms. The northern part of the Madeira Plain is broken by a series of low scarps (Pl. 13, fig. 3). The gradient of the plain between successive scarps is about 1:1500.
The Cape Verde Abyssal Plain occupies 200,000 square miles of sea floor (Pl. 9, fig. 3). The transition from abyssal plain to abyssal hills is gradual. Many hills are scattered in the plain near the boundary. Small scarps of the kind observed in the Madeira Abyssal Plain have not been found.
Recent soundings southwest of Madeira indicate that a low topographic feature which might be called the Madeira Rise continues into the northeast part of the area shown as Madeira Abyssal Plain. This abyssal plain is thus somewhat smaller than indicated in the drawings. Two PDR records from this rise area are illustrated (Pl. 13, figs. 1, 2). Since the limits of the Madeira Abyssal Plain are based on only four sounding profiles, future surveys will cause relatively great changes in this portion of the diagram.
ABYSSAL HILLS
Definition and distribution.—An abyssal hill is a small hill that rises from the ocean-basin floor and is from a few fathoms to a few hundred fathoms in height and from a few hundred feet to a few miles in width. The term abyssal hills province is applied to those areas of the ocean-basin floor in which nearly the entire area is occupied by hills—that is, the province lies at approximately the depth of the adjacent abyssal plain but lacks a smooth floor. Isolated abyssal hills and groups of abyssal hills also occur in the abyssal plains.
Figure 25.—Index chart, location of abyssal hills profiles illustrated in Figures 26 and 27
Limits of abyssal hills provinces shown by dotted lines.
