Outwash terraces at the confluence of the Avon and Spey at Bridge of Avon
Braided channel pattern, with gravel bars and branching channels
Imbrication of flat pebbles: a property perhaps first recognised in 1860 by T F Jamieson
Outwash terraces
Definition: outwash is the meltwater that flows from the glacier. It is often highly charged with sediment and this may accumulates down valley. River incision shapes this stratified sand and gravel into sequences of terraces.
Many of the Cairngorm glens contain stepped sequences of outwash and river terraces. The best known are in Glen Feshie (Young, 1975; 1976). Here the terraces are related to the retreat of ice into upper Glen Feshie and Glen Tromie. The progressive development of soils on late-glacial outwash and younger river terraces has been traced here (Robertson-Rintoul, 1986).
A good example of an outwash terrace can be seen transected by the gravel pit at Inverdruie. Here a valley sandur or outwash plain existed just as the last ice was melting in Glenmore. The quarry sections show horizontally bedded gravels and sands - and the braid bars of the original floodplain can be seen.
In the lower photo, the flow direction towards the left can be judged from the imbrication of the gravel. Clasts tend to be stacked like bricks in the direction of former flow.
