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The treads of river terraces are formed by processes analogous to those that produce floodplains. In depositional terraces, however, the origin of the now abandoned floodplain is much less significant than the long-term episode of valley filling that preceded the final embellishment of the tread. The thickness of valley-fill deposits is much greater than anything that could be produced by vertical accretion on a floodplain surface. In fact, most of the valley fill is composed of channel deposits rather than floodplain deposits. Thus, the sediment beneath a depositional terrace reflects a continuously rising valley floor. The tread represents the highest ...(100 of 32811 words)