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. 50
