Pedimentation

Pediments, the term being used in a broad sense, and not glacial features, are the most striking landscape forms to be seen in the middle part of the Wright Valley. Pediments are essentially a flattish surface with a knick separating it from a steeper surface above. The knick is the critical part. It is related to the way in which rock waste is broken down. To produce a knick the material has to be broken down to a size that can be transported away and the breaking down has to be done in a single step, from large to fine and without appreciable material of intermediate sizes. Salt fretting does this very thing, breaking rocks at one step into their component crystals. At Lake Bull, the name given to a small lake in the Wright Valley, about a mile east of the east end of Lake Vanda, pedimentation is actively in progress, salts being supplied during the flooding of the Onyx River, and the fretted material being wind transported. The flooding carries the salts away from all except the edge of the lake and prevents salt fretting from pitting the lake bed itself. Higher pediments are also attributed to salt fretting, but the controlling details are less well understood.