In general the coarser sediments are lodged near shore and those farther from the land become progressively finer. Even the coarser part of the material carried in suspension by the undertow is partly left in the shallow water. On the other hand, waves of exceptional strength may carry coarse material into water of some depth. Thus coarse shingle (gravel) and even bowlders have been found at depths of 10 fathoms.[173] Coarse deposits may extend far out from land if the waves are strong, and especially if the water is shallow, and since the zone of shallow water may be extended seaward by the aggradation of the bottom, shallow-water deposits may cover extensive areas. They may become deep at the same time, for as the outer border of the shallow-water zone is shifted seaward by aggradation, the vertical space to be filled becomes greater (compare Figs. 321 and 322). Again, if the coast be sinking, new deposits of coarse material may be made on older ones. In this way also great thicknesses of sediment may be accumulated, all parts of which were deposited in shallow water. The great thickness of some of the conglomerate beds of the past shows how far this process may go. [Illustration: +Fig.+ 324.—Ripple-marks.] [Illustration: +Fig.+ 325.—Rill-marks resembling impressions of seaweeds. Beach at Noyes Point, R. I. (Walcott, U. S. Geol. Surv.)] [Illustration: +Fig.+ 326.—Rill-marks. Same locality as 325. (Walcott.)] As a rule, no definite line marks the seaward terminus of the coarse detritus, since coarse material is carried farther out when the waves run high (and the undertow is strong) than when they are feeble. In calm weather, therefore, fine sediment may be deposited where coarse had been laid down in the preceding storm, only to be covered in turn by other deposits of a different character. Thus gravel grades off into sand, with more or less overlapping or interwedging, and sand grades off into silt in the same way. This is diagrammatically illustrated by Fig. 323.