Page:VCH Norfolk 1.djvu/47

 GEOLOGY GLACIAL DRIFT The succeeding strata of Lower Boulder Clay, comprising beds of stiff unstratified Till with intermediate laminated muds, tell of Glacial con- ditions when ice extended over the area and temporarily retreated during the intervals when the glacier mud was spread over a limited area. The Till itself contains much Chalk and many glaciated erratic stones. The softer chalky material is often crushed, the deposit having a streaky fluxion-structure, regarded by Mr. Reid as due to the sliding pressure of an ice-sheet. Blocks from Scandinavia, such as the ' Rhomben porphyr,' have been identified. The Lower Boulder Clay is best seen in the cliffs at Happisburgh, Bacton and Mundesley. It is overlaid by a mass of contorted loams and sands, the disturbances in which form one of the remarkable features in the ' mud cliffs ' of Cromer. Fragments of Tellina balthica, Cardium edule, Cyprina islandica and Mya arenaria occur in the loams, and more abundantly in the sands. Between Overstrand and Sheringham the beds contain huge incorporated strips or boulders of Chalk — one mass measur- ing 500 feet in length.' In this remarkable instance, as in some other cases, the flint-layers in the Chalk masses were comparatively undisturbed. Elsewhere the masses of Chalk have been crushed, the flints fractured, and the fragments scattered. Again, in other instances the Chalk has been ground up and intermixed with clay, and it forms huge masses of marl which are dug for lime-burning, the lime being well suited for agri- cultural purposes. Many old pits also occur in the fields, whence the marl was formerly dug to put on the land. As we pass from east to west, so the Contorted Drift (as it is called) becomes less loamy and more and more marly. In East Norfolk the calcareous loam of Happisburgh and Bacton, which yields a rich soil, constitutes some of the best agricultural land in Norfolk. It extends inland to Ludham, Tunstead, Plumstead and Hamlington. Near Nor- wich, where it is known as the Norwich brickearth, it has been largely worked for brick-making on the borders of Mousehold. In older times it was dug near Markshall, where traces have been found of a Roman kiln situated not far from Caistor Camp. Large boulders of basalt, quartzite and other rocks have been occasionally encountered in agricultural operations on the loamy subsoil of the Contorted Drift, and the blocks have been removed to adjacent villages and homesteads, where they are placed alongside buildings for protection or for use as horse-blocks. On the coast where the cliffs of Contorted Drift contain great masses of sand, the winds, and especially the easterly and north-easterly breezes, blow away the sand and accelerate the destruction of the cliffs. Adjoin- ing the coast near Cromer and Runton, as Mr. Reid has remarked, occasionally the whole of the top soil of a field may be blown away, ' See Lyell, ' On the Boulder Formation, or Drift and associated Freshwater Deposits composing the Mud-clifis of Eastern Norfolk,' Phil. Mag., ser. 3, vol. xvi., 1840, p. 345. I 17 C