The lower courses of many rivers, including most of those in the northern United States, and some in the southern, are bordered by a succession of terraces that rise symmetrically on the two sides of the stream. Sometimes, as in many English rivers, the terraces are at different levels on opposite sides. The formation of these terraces is due to a twofold activity of the river; the combined deepening of the channel and building up of the flood plain at length make the trough of the river so deep that floods no longer fill it, especially if the velocity of the current be maintained or increased by an elevation of the region drained by the river. Then the energy of the current is partly employed in widening the channel and forming a new flood plain, cutting back the edges of the old flood plain, which it can no longer overflow, thus converting it into a terrace, which is the remnant of an old flood plain. The process may be repeated many times, and thus successive terraces rise, one above another, as we recede from the river.

It necessarily follows from this account that the highest terrace is the oldest, and the lowest is the last formed. This seems to be a violation of the rule that, in any series of sedimentary deposits, the oldest must be at the bottom and the newest at the top; but the violation is only apparent, not real. Were the river to flow at a constant level, no terraces could be formed, and the deposits would follow the rule, just as they do now in each successive flood plain and terrace. Because, however, the stream flows at successively lower levels, the lower flood plain is made up of the newer deposits. It should further be observed that the older gravels do not actually overlie the newer ones, but are merely at higher levels.

Fig. 97. - River terraces; Chelan River, Wash. (U. S. G. S).

Unsymmetrical terraces, which are either confined to one side of the river, or if present on both sides, are on different levels, are formed when a stream is widening its valley by steadily cutting away the bank on one side, shifting the channel toward that side, and at the same time deepening it. This will result in the formation of terraces representing the former positions of the stream. If the lateral movement be all in one direction, the terraces will all be on the side away from which the channel is shifting; if it be alternately in opposite directions, terraces will be formed on both sides, but at different levels.

Another method of terrace formation should be mentioned. If a river which has excavated a deep valley, have its velocity checked by a slow subsidence of the country, it will commence to fill up its valley with gravel or other sediment, and may thus accumulate material of great thickness and extent. Should a re-elevation of the country now occur, the river will acquire new destructive power and cut a terraced channel down through its own deposits. In such a case the material is a continuous mass, and the gravels of the higher terraces are newer (not older) than those of the lower. The rivers Mersey and Irwell in England are believed to be examples of this mode of terrace formation.

Fig. 98. - Terrace on deserted channel; central New York. (U. S. G. S).

Rock terraces in river valleys are the result of erosion, not of deposition, and are due to harder ledges of rock which are exposed by the cutting of the river. Rock terraces may likewise be occasioned by diastrophic movements and by long-period fluctuations in the volume of the stream. In all cases terraces indicate the successive levels at which the river has flowed, but they do not imply, as would seem at first sight to be the case, that the river was once of sufficient volume to fill up the space between and below the terraces.