How stratified deposits are built up is well illustrated in the flats which border estuaries, such as the Bay of Fundy.  Each advance of the tide spreads a film of mud, which dries and hardens in the air during low water before another film is laid upon it by the next incoming tidal flood.  In this way the flats have been covered by a clay which splits into leaves as thin as sheets of paper.

It is in fine material, such as clays and shales and limestones, that the thinnest and most uniform layers, as well as those of widest extent, occur.  On the other hand, coarse materials are commonly laid in thick beds, which soon thin out seaward and give place to deposits of finer stuff.  In a general way strata are laid in well-nigh horizontal sheets, for the surface on which they are laid is generally of very gentle inclination.  Each stratum, however, is lenticular, or lenslike, in form, having an area where it is thickest, and thinning out thence to its edges, where it is overlapped by strata similar in shape.

Cross bedding.  There is an apparent exception to this rule where strata whose upper and lower surfaces may be about horizontal are made up of layers inclined at angles which may be as high as the angle of repose.  In this case each stratum grew by the addition along its edge of successive layers of sediment, precisely as does a sand bar in a river, the sand being pushed continuously over the edge and coming to rest on a sloping surface.  Shoals built by strong and shifting tidal currents often show successive strata in which the cross bedding is inclined in different directions.

Thickness of sea deposits.  Remembering the vast amount of material denuded from the land and deposited offshore, we should expect that with the lapse of time sea deposits would have grown to an enormous thickness.  It is a suggestive fact that, as a rule, the profile of the ocean bed is that of a soup plate,—­a basin surrounded by a flaring rim.  On the continental shelf, as the rim is called, the water is seldom more than six hundred feet in depth at the outer edge, and shallows gradually towards shore.  Along the eastern coast of the United States the continental shelf is from fifty to one hundred and more miles in width; on the Pacific coast it is much narrower.  So far as it is due to upbuilding, a wide continental shelf, such as that of the Atlantic coast, implies a massive continental delta thousands of feet in thickness.  The coastal plain of the Atlantic states may be regarded as the emerged inner margin of this shelf, and borings made along the coast probe it to the depth of as much as three thousand feet without finding the bottom of ancient offshore deposits.  Continental shelves may also be due in part to a submergence of the outer margin of a continental plateau and to marine abrasion.