Page:VCH Suffolk 1.djvu/46

A HISTORY OF SUFFOLK of which are sometimes filled with calcareous veins. Beds of this character have been used for building purposes, as in the tower of Chillesford church ; they have been quarried like a freestone, and perhaps on this account the name 'crag' may have been introduced into East Anglia. Blocks also have been obtained from the ' Thorpe Rocks ' on the beach near Aldeburgh. The stone beds do not yield the rich variety of mollusca found in the loose sands, but this, as pointed out by Mr. F. W. Harmer, is owing to the fact that the shells have been largely dissolved away by the action of acidulated water. Consequently the sub-divisions made in the Coral- line Crag by Prestwich are not to be regarded as successive zones, but rather as altered local conditions in the strata. In proof of this Mr. Harmer has pointed out that at Brick-kiln farm, Iken, a lenticular patch of the shelly sands occurs in the midst of a mass of the indurated beds.^ The Coralline Crag attains a thickness of 50 or 60 feet, and from its pale buff tint it has sometimes been termed the White Crag, in dis- tinction from the Red Crag which overlies it. At some depth below ground all the Crag beds are usually grey in colour. The principal exposures of Coralline Crag are at Tattingstone, south of Ipswich ; at Sutton and Ramsholt, south of Woodbridge ; and at Gedgrave, Sud- bourne, Orford and Aldeburgh. From the abundance of fossils at Gedgrave the formation has been termed the ' Gedgravian ' by Mr. Harmer, and characterized as the zone of Mactra triangnla. Among the more abundant and noteworthy fossils are Cardita senilis, Pectunculus glycimeris, Cyprina islandica, C. rustica, Astarte omalii, Diplo- donta rotundata, Nucula nucleus, Pecten opercularis, P. tigrinus, Trophon consocialis, T'urritella incrassata, Calypraa chinensis. Valuta lamberti, etc. At the base of both Coralline and Red Crag, but chiefly below the Red Crag, there occurs a remarkable nodule and pebble bed which has yielded numerous derived fossils, many of them phosphatized. It is well known as a ' Coprolite bed,' and will be referred to more particu- larly in reference to the Red Crag. It forms a layer 12 to 15 inches thick beneath the Coralline Crag at Sutton, and has there yielded pebbles of quartz, quartzite, flint, septaria from the London Clay, bones of Jurassic saurians, and a large boulder of red porphyry, weighing about a quarter of a ton." Coprolites were worked at this locality for a short period. The most interesting fossils are those enclosed in rolled frag- ments of sandstone and known as ' boxstones.' They include Valuta auris-leparis, Conus dujardini, Nassa conglobata and Isocardia car (and var. lunulata), and these with other forms characterize an older Pliocene deposit, no longer existing in situ in this country. The boxstones, which thus represent remnants of an earlier fauna than the Coralline Crag, have been locally used for road metal. The fauna of the Coralline Crag, as observed by Lyell, indicates a warmer temperature than that of the later stages of the crag. The sea was open to the south, and the mollusca ' Proc. Geol. Asioc. xv. 436, xvii. 424. • Prestwich, Quart. Joum. Geol. Soc. xxvii. 117 ; E. Ray Lankcster, ibid. xrvi. 493. 12