These deposits, judging by the shells which they contain, appear, according to Professor Edward Forbes, to have been formed in a sea of moderate depth, generally from 15 to 25 fathoms deep, although in some few spots perhaps deeper. But they may, nevertheless, have been accumulated at the distance of 40 or 50 miles from land. The Suffolk crag is divisible into two masses, the upper of which has been termed the Red, and the lower the Coralline Crag.[162-A] The upper deposit consists chiefly of quartzose sand, with an occasional intermixture of shells, for the most part rolled, and sometimes comminuted. The lower or Coralline crag is of very limited extent, ranging over an area about 20 miles in length, and 3 or 4 in breadth, between the rivers Alde and Stour. It is generally calcareous and marly--a mass of shells and small corals, passing occasionally into a soft building stone. At Sudbourn, near Orford, where it assumes this character, are large quarries, in which the bottom of it has not been reached at the depth of 50 feet. At some places in the neighbourhood, the softer mass is divided by thin flags of hard limestone, and corals placed in the upright position in which they grew. The Red crag is distinguished by the deep ferruginous or ochreous colour of its sands and fossils, the Coralline by its white colour. Both formations are of moderate thickness; the red crag rarely exceeding 40, and the coralline seldom amounting to 20, feet. But their importance is not to be estimated by the density of the mass of strata or its geographical extent, but by the extraordinary richness of its organic remains, belonging to a very peculiar type, which seems to characterize the state of the living creation in the north of Europe during the older Pliocene era.