596 STRATIGRAPHICAL GEOLOGY 



Mr. C. Reid has pointed out that the proportion of extinct forms,. 

 so far as the fauna has been worked out, is 41 per cent, which 

 exactly corresponds with the proportion in the Coralline Crag ; K 

 and in view of this and of the preponderance of southern Mollusca r 

 he concludes that the St. Erth Beds are of older Pliocene age. 

 Mr. Kobert Bell, Professor Kendall, and Mr. Harmer are, however, 

 all opposed to this view, and consider that the St. Erth Beds corre- 

 spond more nearly with the older part of the Red Crag. From the 

 nature of the deposit, from its geographical position, and from the 

 occurrence of beach deposits round St. Agnes Beacon up to 375 feet 

 above the sea-level Mr. Reid infers that the fossiliferous clay was- 

 formed at a depth of 40 or 50 fathoms. 



Red Crag. Returning now to the eastern counties we come 

 to the deposit which is generally known as the Red Crag, this 

 name denoting the usual reddish -brown colour of the material r 

 which differs considerably from that of the Coralline Crag. The 

 latter (when not decalcified) is a light-yellow calcareous deposit 

 consisting chiefly of organic debris with some fine sand, while the 

 Red Crag is a ferruginous shelly sand, its unaltered portions 

 consisting partly of quartz-sand and partly of shells (either perfect 

 or broken), the whole stained by a variable proportion of peroxide- 

 of iron (from 5 to 16 per cent). 



This formation covers a much larger area than the older 

 (Coralline) crag, and is apparently continuous over a space of about 

 300 square miles in Suffolk, but is so often concealed by the sands 

 and clays of the Glacial Series, that generally it is only on the 

 slopes of the valleys which intersect the district that the crag 

 actually comes to the surface. The best exposures occur in the 

 cliff-sections of Walton-on-Naze, Felixstow, Bawdsey, and in the 

 valleys of the rivers Orwell and Deben. 



The Red Crag rests partly on the London Clay and partly on 

 the Coralline Crag, wrapping round the isolated reefs of the latter, 

 and filling up the hollows between them. At its base there is 

 generally a bed of phosphatic nodules with " boxstones " and other 

 derived pebbles, together with rolled bones and teeth of mammalia 

 and of sharks. The manner in which the Red Crag is 1 tanked 

 against the Coralline Crag is illustrated in the section, Fig. 199. 

 In the pits near Sutton there is evidence of two shore-levels, an 

 upper and a lower cliff-line. The upper cliff is about 1 2 feet high, 

 the lower shore-line is about 9 feet below the upper, and these 

 two shore-lines are traceable all round the small mass of Coralline 

 Crag which evidently formed a reef in the Red Crag Sea. 



From the researches of Mr. S. V. Wood 15 and Mr. F. W. Harmer 16 

 it appears that the crag in the southern part of this area is- 



