Fig. 143.
Section near Ipswich, in Suffolk.
- a. Red crag.
- b. Coralline crag.
- c. London clay.
The position of the red crag in Essex to the subjacent London clay and chalk has been already pointed out ([fig. 142.]). Whenever the two divisions are met with in the same district, the red crag lies uppermost; and, in some cases, as in the section represented in [fig. 143.], it is observed that the older or coralline mass b had suffered denudation before the newer formation a was thrown down upon it. At D there is not only a distinct cliff, 8 or 10 feet high, of coralline crag, running in a direction N.E. and S.W., against which the red crag abuts with its horizontal layers; but this cliff occasionally overhangs. The rock composing it is drilled everywhere by Pholades, the holes which they perforated having been afterwards filled with sand and covered over when the newer beds were thrown down. As the older formation is shown by its fossils to have accumulated in a deeper sea (15, and sometimes 25, fathoms deep or more), there must no doubt have been an upheaval of the sea-bottom before the cliff here alluded to was shaped out. We may also conclude that so great an amount of denudation could scarcely take place, in such incoherent materials, without many of the fossils of the inferior beds becoming mixed up with the overlying crag, so that considerable difficulty must be occasionally experienced by the palæontologist in deciding which species belong severally to each group. The red crag being formed in a shallower sea, often resembles in structure a shifting sand bank, its layers being inclined diagonally, and the planes of stratification being sometimes directed in the same quarry to the four cardinal points of the compass, as at Butley. That in this and many other localities, such a structure is not deceptive or due to any subsequent concretionary re-arrangement of particles, or to mere lines of colour, is proved by each bed being made up of flat pieces of shell which lie parallel to the planes of the smaller strata.
Some fossils, which are very abundant in the red crag, have never been found in the white or coralline division; as, for example, the Fusus contrarius ([fig. 144.]), and several species of Buccinum (or Nassa) and Murex (see [figs. 145], [146.]), which two genera seem wanting in the lower crag.
Fossils characteristic of the Red Crag.
