xxx SUPPLEMENT TO THE CRAG MOLLUSCA. 



* 



The rest of the Post-glacial formations shown in the map and sections under the 

 number 11, are with some exceptions, (among which must be included beds wherein Cyrena 

 fluminalis occurs, such as those at Stutton on the Stour estuary, and at Gedgrave near 

 Orford, regarded by us as belonging to the older part of this period,) probably 

 newer than these marine gravels, and belong to the later part of the Post-glacial period. 

 In the preceding Yorkshire Coast section the earlier Post-glacial series, the Hessle sand 

 and clay (d and e), are excavated or removed to give place to numerous later beds (/) of 

 sand and gravel, which are of considerable thickness, and some of which contain freshwater 

 Mollusca. It is clear from this coast section, therefore, that extensive beds, especially 

 river gravels, accumulated over the north-east of England after the land had emerged from 

 the Hessle clay re-depression ; and to these we consider the principal part of the East 

 Anglian Post-glacial Valley beds, which are shown in the map, under the number 11, 

 belong. 1 



The recent alluvium, shown in the map under the figure (that of a crow flying) which 

 has been adopted in their maps by the Geological Survey to distinguish these deposits, 

 is mainly due to that considerable depression anterior to historical times, which buried 

 so much forest ground all round the English coasts. This last depression brought the sea 

 water into valleys which during the preceding (later Post-glacial) period were dry and 

 forest-covered ; and filling them, has given rise to the Broads of East Norfolk. The same 

 depression has produced the wide flats of alluvium which fill so much of the valleys of the 

 Waveney, Ant, and Yare, in East Norfolk, by silting up the lower parts of these valleys with 

 modern estuarine mud, which was found at Yarmouth, in a well boring, to be 170 feet 

 in thickness. 2 



The rest of the recent deposits consist of shingle, such as the great bank which shuts 

 in the Aide from the sea at Orford, or of blown sand, which at Lowestoft has buried the 

 sea cliff, and with some deserted foreshore has produced new land called the Denes. 



S. V. Wood, Jun. 

 E. W. Harmer. 



February, 1872. 



N.B. — The lithographic map having been reduced to one fourth the original scale, 

 from a survey made on the one inch Ordnance sheets, those who may have occasion to 

 use it for field purposes will find it convenient to employ these sheets, (which are of very 



shading and number as the Post-glacial gravel (10), but it, in fact, passes over that gravel where the cliff 

 is highest (under the Ruins), and where a small portion of the clay No. 9 remains in sitil, but the two 

 patches, numbered 10, capping the southern half of the section, consist entirely of this loam. 



1 Sections of the Crag and Glacial beds along valley sides often show a Post-Glacial gravel over them, as, 

 e.g., Sections IX, X, XI, and XIV. It would, however, be scarcely possible to map all these patches, and if 

 done their representation would obscure that of the older beds. 



2 See Prestwich in ' Quart. Journ. Geol. Soc.,' vol. xvi, p. 449. 



