Consolidation of offshore deposits.  We cannot doubt that all these loose sediments of the sea floor are being slowly consolidated to solid rock.  They are soaked with water which carries in solution lime carbonate and other cementing substances.  These cements are deposited between the fragments of shells and corals, the grains of sand and the particles of mud, binding them together into firm rock.  Where sediments have accumulated to great thickness the lower portions tend also to consolidate under the weight of the overlying beds.  Except in the case of limestones, recent sea deposits uplifted to form land are seldom so well cemented as are the older strata, which have long been acted upon by underground waters deep below the surface within the zone of cementation, and have been exposed to view by great erosion.

Ripple marks, sun cracks, etc.  The pulse of waves and tidal currents agitates the loose material of offshore deposits, throwing it into fine parallel ridges called ripple marks.  One may see this beautiful ribbing imprinted on beach sands uncovered by the outgoing tide, and it is also produced where the water is of considerable depth.  While the tide is out the surface of shore deposits may be marked by the footprints of birds and other animals, or by the raindrops of a passing shower.

The mud of flats, thus exposed to the sun and dried, cracks in a characteristic way.  Such markings may be covered over with a thin layer of sediment at the next flood tide and sealed away as a lasting record of the manner and place in which the strata were laid.  In Figure 150 we have an illustration of a very ancient ripple-marked sand consolidated to hard stone, uplifted and set on edge by movements of the earth’s crust, and exposed to open air after long erosion.

Stratification.  For the most part the sheet of sea-laid waste is hidden from our sight.  Where its edge is exposed along the shore we may see the surface markings which have just been noticed.  Soundings also, and the observations made in shallow waters by divers, tell something of its surface; but to learn more of its structures we must study those ancient sediments which have been lifted from the sea and dissected by subaerial agencies.  From them we ascertain that sea deposits are stratified.  They lie in distinct layers which often differ from one another in thickness, in size of particles, and perhaps in color.  They are parted by bedding planes, each of which represents either a change in material or a pause during which deposition ceased and the material of one layer had time to settle and become somewhat consolidated before the material of the next was laid upon it.  Stratification is thus due to intermittently acting forces, such as the agitation of the water during storms, the flow and ebb of the tide, and the shifting channels of tidal currents.  Off the mouths of rivers, stratification is also caused by the coarser and more abundant material brought down at time of floods being laid on the finer silt which is discharged during ordinary stages.