. Elementary physical geography . ollen. The rate also varies with the amount of sediment; for ifthere is no sediment, there are no tools with which to work,and clear water can do little work except that of solution,which is relatively unimportant. On the other hand, ifthe river is given more sediment than it can dispose of, itcannot cut its channel, but must deposit some of its load inthe valley, as is being done in the lower Mississippi. Themost favorable condition is that of a moderate amount of 244 PHYSICAL GEOGRAPHY. sediment. With the hardness of the rocks there is also avariation; for a
. Elementary physical geography . ollen. The rate also varies with the amount of sediment; for ifthere is no sediment, there are no tools with which to work,and clear water can do little work except that of solution,which is relatively unimportant. On the other hand, ifthe river is given more sediment than it can dispose of, itcannot cut its channel, but must deposit some of its load inthe valley, as is being done in the lower Mississippi. Themost favorable condition is that of a moderate amount of 244 PHYSICAL GEOGRAPHY. sediment. With the hardness of the rocks there is also avariation; for a river cannot cut its channel so rapidly ina hard granite as it can in a soft clay. From this it will be seen, that the rate and kind of workthat a stream is doing, varies greatly according to circum-stances ; and it follows that river valleys must present verydifferent characteristics. Some are narrow, others broad;some deep, others shallow; some have rapid slope, othershave a gentle flow, etc. In carving the land, river erosion. Fig. 125. An oceanic volcanic island, showing a cliff produced by wave action in eating back into the land. is an important agent; but its importance does not dependso much upon the work of cutting it does, as upon the factthat it is the agency by which rock fragments, prepared byother means, are removed from the land. River erosion andweathering are intimately combined in the destruction ofthe land, and in the sculpturing of its surface. Ocean Erosion. — The action of the ocean in eroding, isconfined to the limited area of the immediate coast line; buthere it is often very important. The waves are constantly DENUDATION OF THE LAND. 245 beating on the shore, and battering at the rocks, often withterrific force. Armed with sand and pebbles, and even byits direct action, the wave is able to wear back even thehardest rocks; and in the ocean, islands that were once ofgreat size are now only remnants (Figs. 125 and 195). On the beaches and on the headlan
Size: 1566px × 1596px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthortarrralphsralphstockm, bookcentury1900, bookdecade1900
