A HISTORY OF BERKSHIRE 



wide sense, so as to include the Bracklesham and Barton formations. 



Besides these formations however, as has been stated, the surface 

 is much modified by deposits of POST TERTIARY AGE, which, unlike 

 those already alluded to, may be distinguished as superficial deposits. 1 

 Though nowhere reaching any great thickness, they are often thick 

 enough to be the determining factors in fixing the character of the soil 

 and of the plants that grow on it. This fact has been already mentioned. 

 So that over each of our geological formations there is a considerable 

 portion which differs essentially from the ' bed rock,' so that, for 

 instance, on the high ground above Cumnor, Wytham and Bagley, the 

 Corallian or Kimeridge beds are covered with a pebble drift, largely 

 composed of quartzite, and which therefore must have been brought 

 from long distances. 



In addition to these we have bordering our streams alluvial deposits 

 made up of materials carried down by the streams, and dropped 

 whenever a slackening of the current prevented the matter being carried 

 further. They naturally vary in their character, and may be of gravel, 

 loam, or of a clayey consistence, but their constituents must be similar 

 to the river valley in which they are found. Thus, in the alluvial 

 deposits of the Thames above Oxford the pebbles of the gravels are 

 masses of the Jurassic rocks across which the river has run in the part 

 of its course above that city, and the only foreign elements are such as 

 are derived from gravels of an older date which may have been cut 

 through and reasserted. On this alluvial deposit the vegetation will be 

 much influenced by the composition of the gravel. If much limestone 

 be present we may find, as near Yarnton, the orchid Orchis ustu/ata, 

 growing in a rather unusual situation, and the bell flower (Campanula 

 glomerata), lady's fingers (Anthyllis Fulneraria), the grass Bromus erectus, 

 and ploughman's spikenard (Inula Conyza) may often be found in the 

 gravels made up to a great extent of chalk fragments in the meadows 

 near Windsor and Newbury. The sulphur wort (CEnanthe silaifolia), the 

 water avens Geum rivale and its hybrid G. intermedium, the dock 

 (Rumex maritimus), the snake's head (Fritillaria Me/eagris), the great 

 burnet (Poterium qfficinale], the pearl wort (Sagina nodosa], the sedge 

 Scirpus caricis, and the adder's tongue (Ophioglossum -vulgatum) are 

 characteristic species of the alluvium. 



The high-level alluvium is also largely represented in our area, 

 and as it affords a porous soil, and from its varied composition, it yields 

 an interesting vegetation. 



In some of our valleys peaty deposits are found, and at Newbury 

 they were somewhat extensive. In the Lambourn valley also peat was 

 cut at the beginning of last century, and in such situations the tussock 

 sedge (Carex paniculata) and the water dropwort (CEnantbe crocatd] are 

 sure to be found. 



1 With the exception of some of the alluvia, these deposits are not shown on the Geological Map, 

 but will be found in the ' Drift ' edition of the maps of the Geological Survey. 



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