External Form and Internal Structure.—Plains of stratified gravelly sand, half a mile or more in diameter, standing mesa-like above the adjacent valley ground, are common in many parts of New England. They lie on striated ledges and till, and hence are at most not older than the closing stages of the latest glacial epoch. Their distinct marginal slopes give no indication of more than a small measure of erosion, and hence their present form may be taken as essentially equivalent to their initial constructional form. They are well stratified throughout, and this, along with their definite marginal slope, indicates them to be deposits made in bodies of standing water.
The general surface of these sand plains is very even, but fails of being level by reason of a gentle slope, generally to the south, of ten, twenty, of thirty feet to the mile. Their margins are in most cases . . .