26 Plant-life of the Oxford District 



or the equivalent of 9 inches (varying 4-16) is accounted for by the river- 

 system. This leaves an approximate average of less than 1 8 inches per annum 

 to supply plant-life ; the suggestion being that an annual rainfall of 18 inches 

 would be sufficient for the vegetation of the district, if the river did not run 

 off more at Sand ford than at King's Weir. 1 



Springs and Brooks. Apart from the main river-system of the Thames 

 and its backwaters, canals and tributaries (Cherwell, Ray, Evenlode, Wind- 

 rush, Thame), local conditions of water-supply including small streams and 

 swamp-areas are more conspicuously dependent on actual rainfall and the 

 relation of this to percolation through the soil, and the capacity of the latter 

 for penetration and retention. The geological formation is peculiarly 

 favourable for the throwing out of springs at all levels. As previously 

 indicated, the basal formation of the Oxford Valley is a deep bed of Oxford 

 Clay, which is apparently wholly impervious throughout the greater 

 proportion of its depth. In this clay the present river-system has been 

 eroded over the greater part of the district, the valley being covered with 

 a sheet of alluvium to the limit of older winter-floods, while patches of 

 gravel of older river-terraces are left as irregular deposits and the debris of 

 torrential water-courses. These latter are freely permeable as collecting 

 areas, and water may be thrown out at any outcrop of the clay, at levels 

 corresponding to the gravel deposit on an old clay bottom, at 300 ft. or more 

 elevation. Over the clay, patches of residual beds of Corallian strata of 

 limestones and sands, again more freely pervious, cover wide areas (as at 

 Headington, Cowley, Littlemore, Tubney) with Coral Rag on hill-slopes 

 and broad undulating areas (Hinksey, Headington, Cumnor, Stow Wood). 

 At the exposed junction of these strata with Oxford Clay on the slopes of 

 the hills (Cumnor, Headington, Wytham, Elsfield) springs are thrown out in 

 large numbers, giving streams cutting deeply through to the clay-bottom 

 (Hinksey, Headington Wick, Bullingdon Bog), all tending to feed the main 

 river-system, and also forming characteristic swampy patches on the clay 

 sides of the slopes by producing Juncus, Phragmites, and Equisetum 

 associations, at elevations of 300 ft. or so nearer the city, as a second series 

 of springs. 



Over these more pervious rocks, broad tracts of Kimeridge Clay con- 

 stitute another more or less impervious layer, more particularly on the 

 Western side of the district, as iii areas of Bagley Wood, Boar's Hill, 

 Sunningwell. Locally, again, on the surrounding hills the Kimeridge Clay- 

 is capped by highly pervious patches of Greensand (Cumnor Hurst, Boar's 

 Hill), Portland Beds, and Shotover Sands (Shotover, Garsington, Baldon), 

 so that water percolating through these beds is largely thrown out in 

 surface-springs at the junction with the Kimeridge Clay, at a higher level of 

 about 400 ft., 2 giving an upper zone of springs, and again formation of 

 Hill-swamps of Equisetum, Phragmites^ and Typha> with Juncetum at this 

 higher elevation. Patches of Plateau Gravel over Kimeridge Clay (Bagley), or 

 over Gault Clay (Baldon), also assist in giving rise to similar high-level springs, 

 and affording the water-supply of the population of these areas. Swampy 

 ground with Mosses and Juncetum may obtain at any elevation, giving Juncus 

 where most exposed, Equisetum Telmateia, in vast abundance in hollows or 

 on Hill-slopes (N. Hinksey), 7ypha in standing pools (Chawley), with dense 

 thorn-scrub in neglected tracts (Chawley Hurst, Headington Quarry). ^ 



All such streams cut deep hollows in the steep hill-sides, as indicated 



1 In 1921, with extreme drought and rainfall 13-14 in. only, the Thames Valley used all the 

 water falling on it. The Weir-steps were dry at Teddington in Aug. and Sept., and a man crossed 

 the river at the Weir pool (in waders). Vegetation obviously drew on deeper reserves in the ground, 

 and was very insufficiently supplied by rain and stream. 



2 Horspath, Chilswell. 



