Fig. 192.—Terraces of the Frazier River at Lillooet, B. C. (Calvin.)
STREAM TERRACES.
Stream terraces[90] are bench-like flats or narrow plains along the sides of valleys ([Fig. 192]). They are usually narrow, but sometimes have great length in the direction of the axis of the valley. They originate in various ways.
Due to inequalities of hardness.—Reference has already been made ([p. 140]) to the effect of hard horizontal layers in the development of terraces and terraciform projections on the sides of valleys ([Fig. 120]). Such terraces are the result of differential degradation, and the upper surface of the hard layer marks the lower limit of the terrace, which commonly has a distinct slope toward the stream. Except where interrupted by tributary valleys, such terraces are likely to be continuous in a valley so long as the structure remains the same and the stream sustains the same relation to it. Such terraces would first show themselves in the older part of the valley. The effect of inclination of the hard stratum on the development of such terraces will be readily inferred. Terraces and benches of this sort are not equally distinct at all stages of a valley’s history. For great distinctness, the hard layer should have been exposed long enough to allow the general processes of erosion to have effected considerable differential wear, but not long enough to allow the topographic effects of unequal resistance to be obliterated.
Fig. 193.—Diagram illustrating a distinct terrace and a “second bottom (b),” which may be regarded as a low terrace.
Normal flood-plain terraces.—It has been seen that deposition in a river valley stands in more or less definite relationship to the stage of its development, and that the deposition which leads to the development of an alluvial plain is likely to take place where the higher gradient of the upper course gives place to the gentler gradient of the lower. It has also been seen that as a stream’s history advances, the stretch where the gradient is high recedes up-stream, and that the point which marks the head of active deposition follows. It follows that a river flat or flood-plain normally begins in the lower part of a valley, and works progressively headward, its upper end following, at some considerable distance, the head of the valley itself.
The commoner river terraces are remnants of former flood-plains, below which the streams which made them have cut their channels. It has already been pointed out ([p. 184]) that processes of erosion and deposition work together in the development of flood-plains, and that some flood-plains have but little alluvium ([Fig. 174]), while others owe their origin wholly to stream deposits ([Fig. 175]). It follows that terraces developed from flood-plains may be of rock, of alluvium, or of rock covered with alluvium.
The amount which a river channel must be deepened in order to change the remnants of its flood-plain to terraces cannot be definitely stated. When a channel is so deep that the remnants of a former flood-plain are no longer flooded, they would be called terraces, especially if a lower flood-plain has been developed. Even though not above the reach of floods, they are often called terraces if they are notably above the channel and separated from it by a lower plain. Thus the flat at b, [Fig. 193], would be called a terrace, even though covered by water in exceptional floods; but the flat at c, but slightly above the channel, would hardly be called a terrace.