Tunnel erosion,upward-unsealing terraces,and dynamic interpretation of alluvial deposits

The morphology of alluvial terraces, as well as the facial composition of their sediments, is controlled by a number of factors, whose relative importance continues to be difficult to understand. Detailed studies of river valleys provide models of alluvium development, which differ rather strongly from existing models that postulate regular and universal changes in alluvium composition and structure in the course of its accumulation and further transformation of depositional bodies into a terrace. Based on examples from South Ural river valleys we propose a model of an “upward unsealing terrace,” which forms as a result of above-the-flow (tunnel) erosion.