Till drumlins, notably abundant on the lowland about Boston and the highland near Spencer; morainic hills, extending, e.g.
The glacial drift east of the Missouri river, unlike that of the New England states, is remarkably free from boulders and gravel, except in a few morainic belts.
The morainic belts and other obstructions in the drift plains hem in the waters in the intervening basins and create what are called " glacial lakes," var y ing in diameter from a few yards to several miles.
The till plains, although sweeping in broad swells of slowly changing altitude, are often level to the eye, and the view across them stretches to the horizon, unless interrupted by groves of trees along the watercourses, or by belts of low morainic hills.
The great ice sheets formed terminal moraines around their border at various halting stages; but the morainic belts are of small relief in comparison to the great area of the ice; they rise gently from the till plains to a height of 50, 100 or more feet; they may be one, two or three miles wide; and their hilly surface, dotted over with boulders, contains many small lakes in basins or hollows, instead of streams in valleys.