[Illustration: Fig. 130. North-South Section, Eighty-Five Miles Long, across the Plateau North of the Grand Canyon of the Colorado River, Arizona, showing Retreating Escarpments _O_, outliers; _V_, canyon of the Colorado; _A-H_, rock systems from the Archean to the Tertiary; _P_, platform of the plateau from which the once overlying rocks have been stripped; dotted lines indicate probable former extension of the strata. How thick is the mass of strata which has been removed from over the platform? Has this work been accomplished while the Colorado River has been cutting its present canyon?] The specific gravity of air is 1/823 that of water. How does this fact affect the weight of the material which each can carry at the same velocity? If the rainfall should lessen in your own state to from five to ten inches a year, what changes would take place in the vegetation of the country? in the soil? in the streams? in the erosion of valleys? in the agencies chiefly at work in denuding the land? In what way can a wind-carved pebble be distinguished from a river-worn pebble? from a glaciated pebble? CHAPTER VII THE SEA AND ITS SHORES [Illustration: Fig. 131. Sea Cliff and Rock Bench Cut in Chalk, Dover, England] We have already seen that the ocean is the goal at which the waste of the land arrives. The mantle of rock waste, creeping down slopes, is washed to the sea by streams, together with the material which the streams have worn from their beds and that dissolved by underground waters. In arid regions the winds sweep waste either into bordering oceans or into more humid regions where rivers take it up and carry it on to the sea. Glaciers deliver the load of their moraines either directly to the sea or leave it for streams to transport to the same goal. All deposits made on the land, such as the flood plains of rivers, the silts of lake beds, dune sands, and sheets of glacial drift, mark but pauses in the process which is to bring all the materials of the land now above sea level to rest upon the ocean bed. But the sea is also at work along all its shores as an agent of destruction, and we must first take up its work in erosion before we consider how it transports and deposits the waste of the land. Sea Erosion =The sea cliff and the rock bench.= On many coasts the land fronts the ocean in a line of cliffs (Fig. 131). To the edge of the cliffs there lead down valleys and ridges, carved by running water, which, if extended, would meet the water surface some way out from shore. Evidently they are now abruptly cut short at the present shore line because the land has been cut back. [Illustration: Fig. 132. Diagram of Sea Cliff _sc_, and Rock Bench _rb_ The broken line indicates the former extent of the land.]