Physical Featiwes 23 



lowered the flood-limit. The river runs on a similar gravel bottom several 

 feet deep over the Oxford Clay. In the flood-meadows the alluvium is 

 2-3 ft. deep (Osney) over the gravel, and the gravel 8-12 ft. deep over the 

 clay. The average depth of the stream is 8-10 ft., except where the gravel 

 has been removed for ballast- purposes. Off the main channel, kept clear for 

 navigation, and where the current is reduced by locks, mud accumulates 

 over the gravel as a nidus for many aquatic plants ; but the clearest stretches 

 show plants rooted in the gravel. The minor tributary streams (Bullingdon 

 Brook), blocked with vegetation as the water is reduced in volume, give 

 peaty bottoms from decomposing aquatics (rather than from Sphagnum}. 

 As the gravel bed (10-20 ft.) of the Thames at Tilbury is 70 ft. below sea- 

 level, on Chalk, it is probable that the entire valley has subsided 50-100 ft. 

 since the Terrace Period. 



Soils. 



The peculiar shapelessness of the political county of Oxfordshire suffi- 

 ciently indicates the failure of the ' county ' idea as applied to any general 

 account of its flora, and this is the more intensified in the case of the Oxford 

 district, half of which is across the river and labelled Berkshire ; while it is 

 only 7 miles across (NE.) to Buckinghamshire. Similarly the Oxford Valley 

 is by no means typical for Oxfordshire, which is made up of bare higher 

 ground of the Cotswolds to the N W., and of chalk ranges of the Chilterns to 

 the SE., any more than it is for the predominant chalk ranges of Berkshire. 1 



The county extends over a long series of outcrops of geological forma- 

 tion, as Chalk, Upper Greensand, Gault, Lower Greensand, Kimeridge Clay, 

 Corallian Limestone and sands, Oxford Clay, Cornbrash, Great Oolite, 

 Middle Lias, &c., of which the Oxford Valley takes only the central members, 

 as a general mixture of clays and calcareous soils, again so blended by 

 surface-denudation, the washing of upper strata over the surface of lower, 

 and the great mass of alluvial drift as gravels and clay silt, quite apart from 

 the nature of the basal geological formation, that the latter becomes rela- 

 tively secondary. 2 



As a rule the Alluvium is mainly under grass. Valley gravels give 

 arable land, and ploughed patches of fields apparently on the alluvium 

 (South Hinksey, Binsey) indicate terrace-gravel. The clays, being heavy to 

 work and cold, are again mainly devoted to poor grassland ; or as in Bagley 

 Wood, to trees. The Corallian rocks are predominantly indicated by arable 

 land, as also the Lower Greensand and Shotover Sands, with the Portland 

 Beds and Calcareous Grit. The Coral Rag of Littlemore and Sandford, 

 Cowley, Headington and Beckley, gives the best cultivated land. The 

 woodland flora, however, being more dependent on humus and water- 

 content, shows little variation in woods on clay or on Corallian formation. 



Lime is conspicuously deficient in Plateau Gravels, and in the more 

 sandy soils of hill-tops, from which it has leached out ; especially in the case 

 of the Lower Greensand of hills, Shotover Sand, Portland Beds, and much 

 of the Calcareous Grit. The river gravels of the Terraces, again, are mainly 

 limestone, lime from which leaches out over the Oxford Clay. The alluvium 

 contains a large percentage of organic matter, the more so, in the peaty 

 bottoms of minor streams (Bullingdon Brook, Headington Wick). 3 A few 



1 County Floras, arranged according to the Natural System, in which also the political area was 

 subdivided by physical features of river-areas, came in with the Flora of Middlesex, intended as 

 a London handbook, by Trimen and Dyer (1839). This ^ as proved a facile but disappointing 

 method of dealing with partial floras. 



2 Orr and Morison (1916), Agriculture in Oxfordshire, Soils, p. 171. Parts of Bagley Wood 

 give a soil with 30 per cent, coarse quartz sand, free from lime, of the Boar's Hill Lower Greensand 

 type, mixed with fine clay and pebble drift over Kimeridge clay. 



3 Considerable deposits of peat appear to have been exhausted in search ot cheap fuel, in the 



