No doubt the erosive action of running water appears to have but small effect in a short time, and we are apt, therefore, to underestimate its power.
This erosive action is due to the gravel, sand, and mud carried along by the water.
Give an example of the erosive power of running water.
They often occur in cavities which have been formed by the erosive action of acidulated water, in the way described in pars.
King's River canyon has been sunk to a maximum depth of 6,900 feet in solid granite solely by the erosive action of the river!
A factor of great importance which notably aided the erosive power of the glacier in this case was the existence of an unusual number of large vertical joint cracks in the granite in this local region.
Due to its great weight, the erosive power of the ice was most potent toward the bottom, successive joint blocks were removed, and the valley was thus widened and the sides steepened or even commonly made practically vertical.
Like running water, ice may have considerable erosivepower when it is properly supplied with tools.
The uplift of the peneplain, however, caused great revival of activity of erosive power by the streams, the larger ones of which soon cut through the loose superficial alluvial deposits and then into the underlying bedrock.
The total erosive effect which has been, and is now being, accomplished by ice compared with that of running water is, however, slight.
As a direct result of this profound rejuvenation the Colorado River has had its erosive activity tremendously increased, and it has carved out the mightiest of all existing canyons--the Grand Canyon.
The greaterosive effect of wind-driven sand is relatively close to the ground because the larger and heavier fragments are not lifted to very considerable heights.
They cut directly across mountain ridges because they flowed over the plain out of which these ridges have been carved, and because their own erosive and transporting power have exceeded those of any of their tributaries or neighbors.
Of still greater destructive influence, however, than these solvent agencies in earth and sea, are the erosive agencies of both.
And, if we add to these erosive and solvent agencies on land the erosive and solvent agencies of the sea, we may almost begin to wonder that anything deserving the name of a geological record is in existence at all.
But although the erosive agency of the sea is thus so inconceivably great, it is positively small if compared with erosive agencies on land.
For example, present erosive agencies are lowering the level of the whole Mississippi basin from the Alleghanies to the Rocky Mountains at the rate of a foot in five thousand years.
The opening at Fort Snelling is not Y-shaped, as in gorges where there has been indefinite time for the operation of erosive agencies.
At the present rate at which erosive agencies are now acting upon the Alps, their dimensions would be reduced one half in a million years.
As the roof gave way, this was constantly falling to the bottom, where, being exempt from further erosive agencies, it must remain as a gravel ridge or kame.
There are other observers, again, who lay especial stress upon the modifications of the erosive powers of running water, due to the presence of the ice.
One set of streams is continually striving to encroach upon the others, and by capturing their headwaters to reduce their erosive power.
At a still later stage the land surface has been so worn by the eroding forces that the whole process of erosion is slackened, and an uplift must occur before the erosive forces regain their lost strength.
This tongue of ice, either by its own erosive power, or because of the glacial and sub-glacial streams which it produced, wore out a notch in the wall as it crossed, and it is this notch which makes the pass.
The ultimate cause of this shift is doubtless the fact that in Great Britain the rainfall diminishes to the east, so that, generally speaking, the westerly streams have more erosive power than the easterly.
For this reason, and because of the low heat in the combustion of pure nitro-cellulose powder, the erosive action upon the gun was reduced to a minimum.
When the new army rifle was developed, it was found that the smokeless powder then used by the army, containing nitro-glycerin, was so erosive as to destroy the accuracy of the arm when only 1,600 rounds had been fired.
Then the erosive waste exceeds the growth by up-bulging, and mountain decay begins.
As soon as it appears above the water it is attacked by erosive agents.
Finally, the erosive forces triumph and the mountain is clean swept away, leaving only the complexly folded rocks of enormous thickness to mark the place of a former mountain.
May not the deep, narrow valleys of the Selkirks be likewise explained from the more rapid action and greater erosive power of the mountain streams in cutting down their channels?
There can be no doubt, then, that glaciers have an erosive action, and therefore must be regarded as agents of denudation.
These canyons afford striking testimony to the erosive power of running water, of which they are the most wonderful illustration in the world.
In the cut-off tunnel the erosive effect of the stream-rush was even more striking.
The retardation of a glacier by its bed has also been referred to as proving its impotence as an erosive agent; but this very retardation is in some measure an expression of the magnitude of the erosive energy.
Before either ice or water can exert great power as an erosive agent, it must collect in sufficient mass.
To the erosive action here indicated must be added that due to the atmosphere (the severance and detachment of rocks by rain and frost), as affecting the forms of the more exposed and elevated peaks.
It is clearly shown that after certain outflows of lava, quietude reigned for a time; that at last the surface of the rock became cool and that erosive agents broke it up into great masses of loose stones.
Out of this plateau, formed by the uplifting of the plain, has arisen through the active attack of erosive forces the truly mountainous character of the district.
The grazing over these areas by sheep and goats not only exhausts this forest cover, but from the cutting up of the soil and the loosening effect of the thousands of tiny hoofs, the erosive action of the rain becomes disastrous.
The somewhat extensive and ramifying cavern of Kent's Hole is an irregular excavation, evidently due partly to fissures or joints in limestone rock, and partly to the erosive action of water enlarging such fissures into chambers and galleries.
They thus gained new erosive power, and cut back at a much greater rate new and deeper channels; with the result that narrow trenches were cut in the floors of the old gently sloping valleys.
Its side streams will become steeper and have more erosive force, and so will eat back their courses most rapidly until they strike the line of the streams on either side.
The retardation of a glacier by its bed has been referred to as proving its impotence as an erosive agent; but this very retardation is in some measure an expression of the magnitude of the erosive energy.
The vast comparative erosive energy of the Horseshoe Fall comes strikingly into view when it and the American Fall are compared together.
By diffusing the shock of the particle, such substances practically destroy the local erosive power.
He gave a minute account of the manner in which the varieties of form are produced, and referred to the effect which the erosive action thus indicated would have on railway and other works executed on sandy tracts.
The erosive power of a river is vastly augmented by the solid matter carried along with it.
Characteristic of the country are the enormous fissures which divide it, formed in the course of ages by the erosive action of water.
The former would afford object-lessons on the erosive action of cascades; the latter would set him speculating on the causes which could have fashioned that strange portal in the limestone crest of the mountain.
It has long been known that Hamilton Inlet was of glacial origin, the immense basin hollowed out by thiserosive agent being 150 miles in length.
The mouth of the river, which is about one mile wide, is blockaded by immense sand bars, which have been laid down gradually by the erosive power of the river.
When stones are firmly set in moving ice, they then acquire a great erosive power, cutting and wearing down the rocks they are forcibly rubbed against; but this condition never obtains in Lower Nubia, as ice is unknown there.
If the erosive power of the river was so active in that time, it cannot be supposed that it then ceased; it would surely have continued to deepen the bed during the following 3000 years.
This erosion will continue, the channel of the stream becoming ever deeper, until a soil is encountered whose resistance is equal to the erosive action of the water.
Whereas in the mountains it is usually desirable to decrease the erosive action of the water, in the valleys the contrary is the case, as the deposits in the lowlands are as dangerous to life and property as was their abstraction above.
The most economical way of combating such cases is by making use of those points that by their formation arrest the erosive action.
Where the streams are larger, and where the erosive action can not be modified by enlarging the channel, as is the case in many gorges, it is necessary to make an artificial bed for the stream and at times to supplement this by masonry dams.
These falls cause breaks in the action of the water not only in stopping the erosive action in its upward march, but also in checking the velocity of the water.
When the erosive action of the water is already too great, the material carried and then deposited by the stream is often made use of to consolidate banks that are threatened.
Geikie states that the doctrine of the origin of valleys by the erosive action of the streams which flow through them, though it has been credited to various writers, was first clearly taught from actual concrete examples by Desmarest.
He brings out and emphasizes the fact, now so well known, that the erosive action of rain and rivers has formed mountains of a certain class.
It rolls out of the mountains with a force which endless aeons have not weakened to a point where it was incapable of carrying the silt torn down by its erosive actions far out into the sea.
So important is the element of erosion, and so leading is the part played by rivers in erosive work, that McGee would gather all land forms under a classification determined by their drainage systems.
But no rift valley, no depression between the tilted lava blocks, resembles the branching valleys that are produced by the erosive action of running water.
The mountains and plateaus built at various times during the Tertiary and at its commencement have been profoundly carved by erosive agents.
Even when gathered into definite channels, ground water does not have the erosive power of surface streams, since it carries with it little or no rock waste.
How does this fact affect its erosive power on hard rock?
The erosive power of waste-laden currents of air is suggested as the sharp grains of flying sand sting one's face or clatter against the window.
We may infer from this instance how slight is theerosive power of clear water on hard rock.
The basins of the Great Lakes are broad preglacial river valleys, warped by movements of the crust still in progress, enlarged by the erosive action of lobes of the continental ice sheets, and blockaded by their drift.
