It is, however, evident that if the base-level of the rivers remains the same for any great length of time, the whole continent will be denuded down to sea level. Such a plain, base-levelled by river action, is called "an almost plain", or peneplain. If marine erosion has cut a shelf
Fig. 16. Coastal Shelves or Plains of Marine Denudation; one at sea-level uncovered at low tide, and one 600 feet above
at the foot of the cliffs this is called a plain of marine denudation; it is distinguished from a peneplain by the presence upon it of sea-shells and the remains of other marine organisms. Raised beaches, resting on plains of marine denudation, are found all along the south-eastern shores of South Africa up to 1300 ft. above sea-level. If the average level of South Africa be taken at 3000 ft., and it requires 3000 years to remove to the sea material
