626 STRATIGEAPHICAL GEOLOGY 



others from Teesdale and from Cumberland, while a few are identical 

 with rocks in the south and west of Norway. 



In Lincolnshire the basement -beds are nowhere visible, but 

 borings prove that there is always a bed of chalk-rubble at the 

 bottom, from 2 to 8 feet thick, and that this rests on a floor of 

 Chalk which is about 80 feet below sea-level along the present coast- 

 line. The thickness of the overlying deposits is from 60 to 100 feet, 

 and they are of two kinds, (1) a lower clay of a purplish brown 

 colour (the " Purple Clay "), containing stones and boulders both 

 large and small ; (2) an upper clay (the " Hessle Clay ") of dull red 

 colour mottled with ashen grey, containing many small stones, 

 pellets of chalk, and bits of coal, but few large boulders. 



Near the border of the Wolds patches of sand and gravel are 

 frequent, often resembling beach deposits, and containing many 

 marine shells, some of which are perfect, but most are broken. They 

 include Mya truncata, Corbula gibba, Mactra subtruncata, Tellina 

 balthica, T. lata, Cardium edule, Cyprina islandica, Venus gallina, 

 and (at Croxton) Corbicula fluminalis, which is also abundant at 

 Kelsey in Yorkshire. At Kirmington, and about 80 feet above sea- 

 level, an interesting deposit of laminated silk or warp occurs con- 

 taining Foraminifera, Rissoa ulvce, Scrobicularia plana, and other 

 bivalves ; it resembles the warp formed between tide marks on the 

 present coast, and is overlain by 10 or 12 feet of beach shingle. 

 The occurrence of such undisturbed marine silt in association with 

 the boulder-clays and marine gravels is of much importance. 6 



The correlation of the beds above described with those of more 

 southern counties and the relation of the eastern to the western 

 deposits are debated questions, but two opinions have been ex- 

 pressed, and a third view is also possible. (1) The eastern series 

 may be newer than the western (Chalky Boulder-clay), and there- 

 fore later than any deposit in East Anglia ; (2) the eastern series 

 may be the older of the two and equivalent to the Lower Glacial 

 of Norfolk ; (3) the eastern and western clays may be of nearly the 

 same age but formed by different ice-streams, the one coming from 

 the north and north-east, the other from the north-west. 



4. East Anglia 



In the north-east of Norfolk, where the most complete sections 

 of the Lower Glacial Beds are found along the coast, the following 

 succession of deposits can be recognised, but there is no place where 

 they can all be seen in vertical succession : 



The Upper or Chalky Boulder-clay. 



/"Contorted Drifts (loams and sands). 

 Lower Group -| Brown Boulder-clay (Cromer Till). 



[Arctic plant bed and Leda myalls bed. 



