Page:A Treatise on Geology, volume 2.djvu/21
CHAP. VI. 7
(whatever they were) excavated the softer strata, and left the hard prominent in terrace cliffs, as in diag. No. 72.
m. Millstone grit summit resting on shales and grits to l, which is limestone, and projects over s, the subjacent argillaceous beds. The same occurs with each lower ledge of limestone l, which, with the gritstone g, usually found beneath, forms a terrace on the hill sides, above a slope of shale.
A different case occurs in valleys which cross and enter deeply into thick masses of red sandstone, such as occurs at Nottingham, Kidderminster, Bridgnorth, &c. At Bridgnorth, for example, occurs a remarkable triple row of terraces on the east bank of the Severn, which appear decisive as to the successive operations by which changes of relative level of the land and the water which excavated the valley were brought about.
All the terraces represented in the diagram No. 73. are formed on the face of the thick and easily excavated red sandstone; but it is only on the left (east) bank of the Severn that they are conspicuous, because this is the salient angle,—for it is always observed among the common daily effects of inundations, that such terrace--