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 GEOLOGY It is usually sub-angular and somewhat rolled and waterworn, and long exposure to the atmosphere has changed its colour from the black of the ordinary chalk flint to a brown or orange tint. There are also very frequently large numbers of pebbles derived from the Eocene pebble beds. In some places the sand and stones are consolidated by a ferruginous cement into a hard conglomerate, and this has been used as a building stone. The tower of Wokingham church, for instance, is mainly built of this conglomerate. The ferruginous bands in this gravel frequently hold up the water which percolates from the surface of the plateaux, and there are in consequence very often springs at the bottom of the gravel near its junction with the underlying formations. The plateaux vary greatly in height above the sea. Bucklebury Common is 444 feet, Tilehurst just over 343 feet, Mortimer Common 334 feet, Easthampstead Plain rather over 400 feet, and the plateau above Sonning 205 feet above sea level. VALLEY GRAVEL. In the valleys of the Thames and its tributaries there are numerous sheets and patches of gravel which it is convenient to separate from those at higher levels under the term Valley Gravel, though in fact there is every stage of transition between the gravels which form flats on the floor of the valleys and those which, owing to elevation and denudation, have become the capping of plateaux. In many cases patches of gravel are separated from the drift on the floor of the valley, but denudation has not proceeded far enough to form them into plateaux, and they are in the intermediate form of terraces. Near Maidenhead, for instance, there are three such terraces, 1 but the highest of these terraces has in some places already become a plateau, as above Staverton Lodge, for instance. There are flats and terraces of gravel at many places by the side of the Thames and Kennet, and a large part of Reading is built on Valley Gravel. There is very little gravel in the valley of the Lodden until it is joined by the Blackwater, but the sheets of gravel along that river and by the Lodden after the Blackwater has joined it are extensive, giving support to the suggestion already mentioned that the drainage area of the Blackwater was once more extensive than now. In the north-west of the county there is also a certain amount of gravel. Buscot Park, for instance, is on flint gravel on Oxford Clay. The thickness of the gravel is very variable, but is often 20 feet or more in the neighbourhood of Reading. Flint implements have been found in many places and mammalian remains have been recorded from gravel at Reading, Windsor, etc. The gravels at all levels in Berkshire often present a curiously contorted stratification, and probably in many cases this may be due to the action of river ice. Possibly much of our gravel was formed during the period of extreme cold known as the Glacial period, but there is a lack of evidence on this point. The Boulder Clay is not found in or 1 W. Whitaker, ' Geology of London,' Geol. Surt'ey, i. 391. 21