504 PLEISTOCENE. 



Middle Glacial Beds.— The Middle Glacial beds (or Middle Drift of Mr. Wood) 

 consist of gravel and sand, with occasional beds or seams of chalky Boulder Clay, 

 near Hertford. The gravel is largely composed of Chalk flints both subangular 

 and rolled, and pebbles of quartz and quartzite. Its thickness varies from lo to 

 upwards of 70 feet. In some localities sand predominates, in others gravel, and 

 occasionally a good deal of brickearth is associated with the beds. The sand 

 sometimes contains pebbles or grains of Chalk. The term Boulder Sands and 

 Gravels was used by Prof. Prestwich. The gravel in some places contains a great 

 many rolled fossils derived chiefly from Jurassic rocks. The pits at Muswell Hill 

 and Finchley, owing to the labours of Mr. N. T. Wetherell, have yielded a large 

 number of specimens (see p. 506) ; and these fossils occur in many other places, at 

 St. Ives, Somersham, etc. 



On the Norfolk coast large basins or pockets in the Contorted Drift ai-e filled 

 with Middle Glacial sand and gravel, and these features are for the most part due to 

 contortion. (See Figs. 81, 87, 88, pp. 472, 502.) The sands and gravels form 

 portions of the higher grounds near Cromer, and they are well exhibited in the 

 cliffs near Yarmouth and Lowestoft. The sands in places contain shoals of broken 

 shells, and these, more conspicuous in the cliffs at Gorleston, Hopton, and Corton, 

 between Yarmouth and Lowestoft, were noticed by the earlier geologists as 

 re-deposited Crag shells. Thus shells were found at Caistor, near Yarmouth, 

 in 1836, by Mr. John Cunn ; and they have been found also at Billockby, in the 

 same neighbourhood. 1 Messrs. Wood and Harmer have procured upwards of 

 100 species of Mollusca from these sands, and the assemblage is a very curious 

 one.'^ With the exception of one Venus, a Loripes, and some new species, all the 

 shells are found in our Crag deposits ; but while including a number of species 

 which might have come out of the Coralline, Red, and Norwich Crags, yet "not 

 a trace or fragment of most of the common strong shells of the Coralline and Red 

 Crags has occurred." Nearly all the specimens found are more or less rolled, but 

 fragile shells like Anoinia ephippiutn are occasionally preserved. Messrs. Wood 

 and Harmer conclude that the fauna was contemporaneous, although the 

 shells bear evidence of having been shifted and rolled by currents which 

 brought them from some other part of the sea-bottom ; while in their opinion 

 some of the delicate shells may have been transported by floating masses such as 

 seaweed. 



Mr. Clement Reid, who has carefully examined the shells found in the Glacial 

 sands of the Cromer Cliffs, remarks : "It is not improbable that a large proportion 

 of the fragments of the common shells may be derivative ; but near Cromer, 

 as at Yarmouth, the peculiar and characteristic types are the most perfect. 

 Though the commonest forms are Tellina Balthica, Cardiiitii edide, Cyprina 

 Jslaiidica, and J\Iya areiiaria, of the last three nothing but small fragments were 

 seen, and of Tellina Balthica only a few nearly perfect valves ; but the single 

 specimens of Nassa reticosa, Ainviiia, and Dentalium were nearly perfect, as were 

 two or three of Scalana Grcoilandica and Natica Qya-nlandica ? The re-appear- 

 ance of Crag forms may be explained by the submergence of the land to a greater 

 extent than had occurred since the time of the Coralline Crag, thus re-opening the 

 connection with the southern seas, and allowing species long exterminated in this 

 area to again migrate into it." ^ There is nevertheless room for great doubt about 

 the shell fauna being contemporaneous. Traced southwards, these so-called 

 Middle Glacial Sands pass mostly into gravels, which underlie the Chalky Boulder 

 Clay over a considerable part of Essex, and only in two or three places in Suffolk, 

 south of Corton, have fragments of Crag shells been found in the Drift of this 

 portion of the eastern counties. Moreover, near Hertford and other places, seams 

 of Boulder Clay have been met with in gravel beneath the main mass of Chalky 

 Boulder Clay — which gravel has been grouped as Middle Glacial by Mr. Wood,* 

 and this does not support the view of a much milder climate ; while the Ostracoda 



' See also Rose, P. Geol. Assoc, i. 193. 



^ See Supplements to Crag Mollusca (Palaeontograph. Soc). 



^ Geol. Cromer, pp. 93, 94. 



* See Whitaker, Guide to Geol. London, ed. 4, p. 62, 



