58 The Soils of England and Wales 



On the midland Trias there are extensive deposits of 

 drift containing erratics, some coming from the north, 

 and some from the Jurassic and Cretaceous rocks further 

 east. Eastwards this drift passes into the chalky boulder 

 clay, which covers a wide extent of country, reaching 

 northward along the coast into Lincolnshire and York- 

 shire, and occupying wide areas of the surface in south 

 Norfolk, Suffolk, Essex, Cambridgeshire, Bedfordshire 

 and Huntingdonshire. Nowhere however does it come 

 so far south as the Thames valley. This heavy calcareous 

 clay forms wide sheets on the higher ground, and some- 

 times extends to the valleys on a level with the allu\T.um, 

 but it has been largely removed from the valley sides and 

 escarpments. It forms a heavy soil and is largely under 

 cultivation. The natural woodland is of the ash-oak 

 type owing to the relatively high lime-content of the soil 

 (see p. 181) and is treated as (ash-) oak-hazel copse. 



There are numerous other Quaternary deposits of 

 various age and the most various composi- 

 ^^^^ tion, some of glacial and many of unknown 



deposits. origin'. Perhaps the most important of these 



is the " clay-with-flints " already referred to 

 (p. 53), which very frequently covers the chalk plateaux 

 and, being generally non-calcareous, quite alters the 

 character of the vegetation. The derived sands and 

 gravels resting on the chalk, and covering consider- 

 able areas on the borders of Norfolk and Suffolk round 

 Brandon, Thetford and Mildenhall, are largely occupied 

 by heath. 



Recent alluvium, laid down by existing rivers, again 



varies very greatly in composition. In moun- 



auuvium. *^™ country it often consists of coarse sand 



and gravel, while in the flood plains of the 



level country it consists typically of fine clayey silt. It is 



1 There is no evidence of glacial action in south-eastern England 

 south of the Thames valley. 



