xliv FLORA OF BERKSHIRE 



overspreads the Bagshot Sands from Sulhampstead Abbots to 

 Silchester, and is found on the liill-tops eastwards, where it rests 

 on the London Clay at Spencer's Wood Common, on tlie Lower 

 Bagshot Beds at Farley and Heckfield Hills, and on the Braeklesham 

 Beds at Bramshill. Finchampstead Leas and Shinfield Green are also 

 covered with gravels. In fact, the hilly ground of the Bagshot Beds 

 is geneially capped with thick layers of gravels richly stained with 

 iron, which has hardened the lower strata into a conglomerate mass to 

 which the name of 'pan' is locally applied. This hard material 

 influences the character of the plateaux of the district where it 

 occurs. 



These gravels are covered with a rather varied flora. The plants 

 found on them have been already mentioned in connexion with the 

 underlying strata. 



High-Level and liOW^-Level Alluvium. Where rivers flow 

 with a gentle fall across flat country, they are usually margined by 

 tracts of flat meadow land, which are composed of materials carried 

 down by the streams, and dropped whenever a slackening of the 

 current prevented the matter being carried further. Such deposits 

 are known as 'Alluvium.' They may be gravelly, loamy, or clayey. 

 Their component materials are such only as are found within the 

 base of the river which they adjoin. Thus in the alluvial deposits of 

 the Thames about Oxford the pebbles of the gravels are mainly of the 

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

 course above that city, and the only foreigners are such as have been 

 derived from gravels of older date. 



The alluvial deposits which immediately adjoin the streams, and 

 are now in the process of formation, are distinguished as Low-level or 

 Valley Alluvium. But we frequently find patches of deposits, 

 obviously alluvial in character, at various and sometimes considei'able 

 heights above the present stream. The present-alluvial flat is bounded 

 by a sharp rise in the ground ; on mounting this we find ourselves on 

 another spread of flat ground covered by alluvial deposits. Sometimes 

 on ascending the sloping sides of the valley we encounter other such 

 flat patches also covered by alluvium. In fact, the flanks of the valley 

 do not rise with a uniform gradient, but are interrupted at intervals 

 by terraces which on examination are found to be covered by alluvium, 

 and each terrace on one side has a terrace to match on the other side 

 at a corresi3onding height above the present stream. 



It is evident that the stream once flowed at the level of the highest 

 terrace, and was margined by a sheet of alluvium such as overspreads 

 the bottom of the valley. After a while it began to cut down its 

 channel, and in doing this swept aw'ay the great part of this alluvial 

 flat, leaving only the fragments which now survive in the terrace. 



