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A narrow body of outwash confined within a valley is called a valley train. A valley train is the area where stream action has builtstratified deposits of gravel, sand, silt, and clay beyond the glacier itself. The sediment is characteristically coarse grained and well sorted. The bedding (if present) is usually thin and truncated by younger beds. Grain size variations are sharp and numerous in both the vertical and horizontal direction. Close to the glacier boulders, gravel, and sand predominate which indicate continual deposition of the bed load. Further from the glacier the grain size decreases with more silt and clay present; filling of glacial lakes is also a type of outwash (see filled lakes).
Because the deposition is confined to a valley, filling occurs more rapidly near the center than along the sides which often causes streams to be dammed and ponded. The small lakes which are created will be filled rapidly with sand and gravel.
The surface of the valley train can be either pitted or nonpitted. In areas where the outwash deposited over an unglaciated area or area not recently glaciated, the surface will tend to be nonpitted. If, however, buried ice blocks are included in the valley train, the ablated blocks will result in a pitted surface marked by numerous kettles. This pitting will be extremely developed in areas of deposition on a stagnant ice mass of irregular thickness. Braided streams are also very common.
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