Sand deposits usually take place in relatively shallow water, and the sand is subjected to much shifting before it finds a permanent lodgment. In the course of this shifting, bars are formed which usually have a rather steep face in the direction in which they are being shifted. The sand carried over the top of the bar finds lodgment on the sloping terrace face. The inclined laminæ thus formed constitute a kind of bedding, but since its planes do not conform to the general horizontal attitude of the formation as a whole, it is called false- or cross-bedding or, more accurately, cross-lamination (see [Fig. 368]). The same structure is developed on delta fronts and generally in water shallow enough to be subject to frequent agitation at the bottom. Sandstone is cross-bedded more commonly than other sorts of sedimentary rock.
The bedding of conglomerate is due chiefly to variations in coarseness. Laminæ or thicker layers of sand are frequently found between layers of coarser material. Conglomerate is likely to be thick-bedded, and cross-bedding is common.
Lateral gradation.—When the varying nature of the agitation of the sea at different depths and along the different parts of the coast-border, and during different phases of the sea-currents, is considered, it will be readily understood that sedimentary beds are affected by many irregularities, and that deposits of one kind grade into others horizontally with great freedom. Thus a bed of conglomerate (gravel) may grade laterally into sandstone, and this into shale or limestone. It is indeed rather more remarkable that the sedimentary strata should be as regular and persistent as they are, than that they sometimes grade into one another.
Fig. 368.—Cross-bedding in sandstone. Dells of the Wisconsin near Kilbourn, Wis. (Bennett.)
Special markings.—The rhythmical action of waves gives rise to undulatory lodgment, known as ripple-marks ([Fig. 324]). They are usually not the direct product of the surface-waves, since they are much too small. They are produced mainly by the vibratory movement of the undertow, but they apparently result from various other phases of vibratory agitation of the bottom waters. They are sometimes made by streams and stream-like currents. Ripple-marks are apparently preserved indefinitely under proper circumstances. They are sometimes found, for example, on very ancient quartzites. Ripples are also made by wind ([p. 37]). Ripple-marks are usually only an inch or two from crest to crest, but in rare instances they attain much greater size. Examples of ripple-marks 30 feet across are known.[211] Occasional ridges and depressions of much greater dimensions are produced which are attributable to the formation of successive bars, or to the building of wave-cusps.[212] Rill-marks are not infrequently produced by the undertow and other currents passing over pebbles, shells, etc. (Figs. [325] and [326]).
Fig. 369.—Mud-cracks in Brunswick Shale, N. J. (Kümmel.)
Sediments are sometimes exposed between tides, or under other circumstances, for periods long enough to permit drying and cracking at the surface. On the return of the waters, the cracks may be filled and permanently preserved. These are known as sun-cracks or mud-cracks (Figs. [328] and [369]). They chiefly affect shales, but are occasionally seen in limestones and fine-grained sandstones. During the exposure of the sediments a shower may pass and rain-drop impressions ([Fig. 370]) be made which are subsequently filled by fine sediment and preserved. The size and depth of rain-drop impressions give some hint as to the meteorological conditions of far-off ages. Wave-marks, which consist of the faint line-ridges developed on a sandy beach at the limit of the incoming wave, are sometimes preserved and may be seen occasionally on layers of rock deposited millions of years ago.
