Fig. 305.—Steep cliff developed by waves. Allen Point, Grand Island, Lake Champlain. (Perry.)
Fig. 306.—Cliff in unconsolidated material (bowlder clay), with lake-beach in foreground. South Manitou Island, Lake Michigan. (Russell, U. S. Geol. Surv.)
Fig. 307.—Steep cliff in unconsolidated material, the result of rapid cutting. Southeast extremity of Grove Point, Md.
Topographic Features Developed by Wave-erosion.
Fig. 308.—Standing Rock. A wave-erosion monument. West shore of Random Sound, south of Clarenville, N. F. (Walcott, U. S. Geol. Surv.)
The sea-cliff.—The action of the waves, cutting as they do along a definite horizontal zone, has been compared to the action of a horizontal saw. As the waves cut into the shore at and near the water-level, the material above, being unsupported, falls, leaving a steep face above the line of cutting. This steep face is known as the sea-cliff ([Figs. 301 to 306]). The same term is sometimes applied to the cliffs of lakes. The principles involved in the development of the sea-cliff are applicable to any broad stretch of water.
The height of the cliff depends on the height of the land on which the sea is advancing. Its slope may be steep or gentle (compare [Figs. 303 to 306]), according to the nature of the material of which it is composed and the rapidity of the cutting. Rapid cutting tends to produce steep cliffs and slow cutting gentle ones, for in the latter case weathering is more important relative to the cutting, and at sea-level (low altitudes) weathering generally tends to reduce the angle of slope. In general, the more resistant the material the steeper the slope of the cliff. Incoherent materials, such as sand and clay, are not likely to form steep cliffs; but if the cutting be very rapid, bold faces may be developed even in such materials ([Fig. 307]). If beds of slight resistance at sea-level underlie beds of greater resistance, the development of steep cliffs is favored. The structure of the cliff-rock also has an influence on the slope. The rock may be massive or bedded. If bedded, the beds may be horizontal, or they may dip at any angle, in any direction. The rock, whether stratified or not, may be abundantly or sparsely jointed. All these structures influence the slope and configuration of the sea-cliff (see [Figs. 305 to 308]).