Some of the stiff boulder clays or " till " so prevalent over parts of the north of England appear to have been deposited from ice sheets during the glacial period.
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.
The morainic belts are arranged in groups of concentric loops, convex southward, because the ice sheets advanced in lobes along the lowlands of the Great Lakes; neighboring morainic loops join each other in re-entrants (north-pointing cusps), where two adjacent glacial lobes came together and formed their moraines in largest volume.
When the ice sheets fronted on land sloping southward to the Ohio, Mississippi and Missouri rivers, the drift-laden streams flowed freely away from the ice border; and as the streams, escaping from their subglacial channels, spread in broader channels, they ordinarily could not carry forward all their load; hence they acted not as destructive but as constructive agents, and aggraded their courses.
It must therefore have been a sort of oasis, when the ice sheets from the north advanced past it on the east and west and joined around its southern border.