Terraces developed by the normal activities of a stream are always low, and it is improbable that they would ordinarily be conspicuous. The vertical distance between the first (highest) and second is greater than that between the second and third. The principles developed on page 65 et seq., in connection with the erosion of the hypothetical island, are applicable here.
Flood-plain terraces due to other causes.—Certain other causes, accidental rather than normal to a stream, result in the development of terraces from flood-plains. (1) If there be uplift in a region where the rivers have flats, the streams are rejuvenated, and the remnants of their former flood-plains become terraces. (2) If an alluvial flood-plain has been built as the result of an excessive supply of sediment ([p. 186]), the exhaustion or withdrawal of the excessive supply would leave the stream again relatively clear, and free to erode where it had been depositing. It would forthwith set to work to carry away the material which it had temporarily unloaded on the plain. The plains built up in many valleys in the northern part of our continent during the glacial period, when the drainage from the ice coursed through them, have subsequently been partially destroyed by erosion, and their remnants have become terraces. A notable reduction in the amount of available sediment, even when the earlier supply was not excessive, produces a similar result. (3) A notable increase in the volume of a stream, without corresponding increase in load, as when one stream captures another, may occasion the development of terraces by allowing the stream to deepen its channel. (4) Above any barrier which dams a stream, a flood-plain is likely to be developed. When the barrier is removed the stream will cut more or less deeply into the plain above, leaving terraces. (5) The recession of a falls through a flood-plain converts such parts of it as remain, into terraces.
In conclusion, it may be stated that many river terraces, mostly very low, are normal features of valley development, coming into existence at definite stages in a valley’s history. They are generally composed, in large part, of river alluvium. Others result from more or less accidental causes, working singly or in conjunction, and to this class belong all of the more conspicuous terraces developed from flood-plains. The structure of a terrace often affords some clue to its origin ([Fig. 196]).
Fig. 196.—Terraces partly of rock and partly of alluvium. Such terraces indicate successive uplifts, or some other change which had a similar effect on the stream which made the valley.
Discontinuity of terraces.—When a stream sinks its channel into its flood-plain, it does not follow that a terrace remains on each side. Where the stream’s deepened channel is in the middle of its flood-plain, there is, temporarily, a terrace on either side; but wherever the deepened channel is at one margin of its flood-plain, a terrace remains on the other side only. Even where continuous at the outset, terraces soon become discontinuous, for all processes of subaërial erosion conspire to destroy them. A stream is likely to meander on its second and later flood-plains, as on its first and highest one. Wherever the meanders on its second flood-plain reach the borders of the first flood-plain, the terrace at that point disappears, and since the meanders are continually migrating, terraces are continually disappearing. The same would be true of the second terrace, if a second were developed. The removal of portions of a terrace by the sweep of meanders is likely to leave the remnants cuspate toward the stream.[91] Again, tributary streams, in bringing their channels into topographic adjustment with their mains, cut through the terraces of the latter. New gullies develop on the faces of the terraces and their heads work back across them, dissecting them still further. At the same time, sheet erosion and other phases of slope wash tend to drive the scarps of the terraces back toward the bluff beyond. By the time a second series of terraces is well developed, no more than meagre remnants of the first may remain.
From the foregoing considerations it is clear that the extent to which river terraces once developed, now remain, is dependent in part on the length of time which has elapsed since the river sank its channel below them. Other things being equal, the greater their age the more meagre their remnants.
Terraces developed from river plains formed chiefly by alluviation stand a better chance of long life than most other alluvial terraces. This is because of the configuration of the original valley, the aggradation of which gave origin to the plain. The principle involved is illustrated by [Fig. 197]. In developing the second flood-plain the river encounters the rock wall of the valley. This greatly retards lateral erosion, and the terrace above, defended[92] by the rock, is likely to be long-lived.
Fig. 197.—Diagram to show why certain terraces are longer lived than others.