Till is, then, characterized by a noteworthy lack of homogeneity, both as regards the size and the composition of its constituent parts. As many as twenty different rock types of varied textures and colors may sometimes be found in a single exposure of this material, and the entire gamut is run from the finest rock flour upon the one hand to bowlders whose diameter may be measured in feet (Fig. 335). In contrast with those derived by ordinary stream action, the pebbles and bowlders of the till are faceted or “soled”, and usually show striations upon their faces. If a number of pebbles are examined, some at least are sure to be found with striations in more than one direction upon a single facet. As a criterion for the discrimination of the material this may be an important mark to be made use of to distinguish in special cases from rock fragments derived by brecciation and slickensiding and distributed by the torrents of arid and semiarid regions. Inasmuch as the capacity of ice for handling large masses is greater than that of water, assorted drift is in general less coarse, and, as its name implies, it is also stratified. From ordinary stream gravels, the kame gravels are distinguished by the form of their pebbles, which are generally faceted and in some cases striated. In proportion, however, as the materials are much worked over by the water, the angles between pebble faces become rounded and the original shapes considerably masked. =Features into which the drift is molded.=—Though the preëxisting valleys were first filled in by drift materials, thus reducing the accent of the relief, a continuation of the same process resulted in the superimposition of features of characteristic shapes upon the imperfectly evened surface of the earlier stages. These features belong to several different types, according as they were built up outside of, at and upon, or within the glacier margin. The extra-marginal deposits are described as _outwash plains_ or _aprons_, or sometimes as _valley trains_; the marginal are either _moraines_ or _kames_; while within the border were formed the _till plain_ or _ground moraine_, and, locally also, the _drumlin_ and the _esker_ or _os_. These characteristic features are with few exceptions to be found only within the area covered by the latest of the ice invasions. For the earlier ones, so much time has now elapsed that the effect of weathering, wash, and stream erosion has been such that few of the features are recognizable.