The constant but unequal waste of glaciers has already been referred to. The streams to which this gives rise are usually laden with gravel, sand and silt derived from the ice. Since the mud is often light-colored, the streams are sometimes described as “milky.” Where the amount of material carried is great, much of it is dropped at a slight distance from the ice, the coarsest being dropped first. Glacial streams are, as a rule, aggrading streams, and therefore develop alluvial plains, called _valley trains_ (Fig. 281 and 282), or where they enter lakes (Fig. 283), bays, or other streams, deltas. In its transportation, the river-borne drift is assorted; after its deposition, it is stratified. True glacial deposits in the upper part of a mountain valley are, therefore, often continued below by glacio-fluvial deposits derived from the same source. [Illustration: +Fig.+ 281.—Alluviation by glacial stream: Nicolai Creek, Alaska. (Schrader, U. S. Geol. Surv.)] The most common form of such deposit is a _valley train_ (Fig. 281) of glacial wash stretching indefinitely down the valley. The silt, sand, and gravel of such trains can usually be distinguished from valley deposits of non-glacial origin by the character of the material, as much of it is the product of grinding, crushing, and fracture, rather than of ordinary surface decay. Its materials are, therefore, fresh and often include rock material which, if long exposed at the surface, would be decomposed or dissolved. [Illustration: +Fig.+ 282.—Alluviation by glacial stream below Hidden glacier, Alaska. (Gilbert, U. S. Geol. Surv.)] Where an ice sheet ends in a broad face, as did the ancient continental glaciers, numerous streams flow from it and spread their débris in front of the terminal moraine, forming a broad fringing sheet or “apron” (_outwash plain_) along it. Where streams of considerable size form tunnels under or in the ice, these may become more or less filled with wash, and when the ice melts the aggraded channels appear as long ridges of gravel and sand known as _eskers_ (_osars_ and _serpentine kames_ and _kames_ of authors. See chapter on glacial period). It has been thought that similar ridges are sometimes formed in valleys cut in the ice from top to bottom, and even that they arise from gravel and sand lodged in superglacial channels. The latter at least is probably rare, as the surface streams usually have high gradients, swift currents, and smooth bottoms, and hence give little opportunity for lodgment. In the case of ice-sheets, too, in connection with which eskers are chiefly developed, there is usually no surface material except at the immediate edge, where the ice is thin and its layers upturned.