In the finer sands and muds of sheltered tidal flats may be preserved the impressions from raindrops or of the feet of animals which have wandered over the flat during an ebb tide. When the tide is at flood, new material is laid down upon the surface and the impressions are filled, but though hardened into rock, these surfaces are those upon which the rock is easily parted, and so the impressions are preserved. In the sandstones of the Connecticut valley there has been preserved a quite remarkable record in the footprints of animals belonging to extinct species, which at the time these deposits were laid down must have been abundant upon the neighboring shores. Between the tides muds may dry out and crack in intersecting lines like the walls of a honeycomb, and when the cracks have been filled at high tide, a structure is produced which may later be recognized and is usually referred to as “mud-crack” structure. This structure is of special service in distinguishing marine deposits from the subaërial or continental deposits. A variation in the direction of winds of successive storms may be responsible for the piling up of the beach sand in a peculiar “plunge and flow” or “cross-bedded” structure, a structure which is extremely common in littoral deposits, though simulated in rocks of eolian origin. =The order of deposition during a transgression of the sea.=—Many shore lines of the continents are almost constantly migrating either landward or seaward. When the shore line advances over the land, the coast is sinking, and marine deposits will be formed directly above what was recently the “dry land.” Such an invasion of the land by the sea, due to a subsidence of the coast, is called a transgression of the sea, or simply a _transgression_. Though at any moment the littoral, shoal water, and offshore deposits are each being laid down in a particular zone, it is evident that each must advance in turn in the direction of the shore and so be deposited above the zones nearer shore. Thus there comes to be a definite series of continuous beds, one above the other, provided only that the process is continued (Fig. 18). At the very bottom of this series there will usually be found a thin bed of pebbly beach materials, which later will harden into the so-called _basal conglomerate_. If the size of the pebbles is such as to make possible an identification, it will generally be found that these represent the ruins of the rock over which the sea has advanced upon the land. [Illustration: FIG. 18.—Diagram to show the order of the sediments laid down during a transgression of the sea.]