Physical Features 13 



Geology. 



Oxford Clay : The beginnings of plant-life in the Oxford district, so 

 far as available, may be said to go back to the horizon of the Oxford Clay, 1 

 which forms a deep pan under the city and immediate neighbourhood, 

 200-400 ft. thick in places, apparently impervious to water, and covering 

 Lower Oolitic strata, the outcrop of which comes to the surface away to the 

 west on the Witney side, with the nearest outliers on the north, as the 

 Cornbrash of Islip and Kidlington. 2 



This clay represents fine silty material laid down as subaerial denuda- 

 tion of the continental fringe, in shallow and warm sea-water, probably in 

 broad, more or less land-locked lagoons of estuarine nature, as indicated by 

 the massive oyster-like Gryphaea dilatata, which may lie in banks or as 

 thickly distributed dead shells. 3 The conditions of a warm sea are also 

 illustrated by the presence of Ammonites and other Cephalopods (Belemnites). 

 The clay is remarkably uniform in texture in the upper levels more com- 

 monly exposed, and the blue colour is due to ferrous sulphides which oxidize 

 on exposure to yellow-browns of ferric oxides. This probably indicates the 

 wastage of an adjacent volcanic land-area, as iron is practically negligible in 

 the water of open seaboard. 



At the base of the clay the material becomes more shaly (Kellaway 

 beds), of hard sandstone with calcareous debris, occasionally bituminous with 

 traces of lignite and iron, as the indication that the clay-deposit followed 

 a depression of previous forest-land, with algal fringes, sinking and gradually 

 covered with coral-reefs in shallower areas. Similar traces of lignite have 

 been found in the deep boring (Brewery, 1898) at depths of 375, 396, and 

 406 ft. beneath the City in similar shales and limestones of estuarine beds 

 of the Lower Oolite. 



This first vegetation of the Jurassic (Mesozoic) may have been of any 

 tree-forms of the present world, Gymnosperm or Angiosperm ; since at the 

 time of the Upper Cretaceous plant-life of the world was to all Botanical 

 purposes as good as it is at the present day. Angiosperm forest timber- 

 trees, 4 flowers with syncarpous ovaries, 5 occur in Cretaceous deposits ; winged 

 land-insects in the Jurassic, even a butterfly. The time required for their 

 evolutionary progression to this level is still beyond computation ; but is 

 probably to be estimated in hundreds of millions of years. 



Though this great bed of blue clay constitutes so bold a feature in the 

 local succession of strata, it represents but one of many long-continued 

 periods of depression and elevation, following on from the older Lower 

 Oolitic sea-floor, as the land on the fringing margin (Continental shelf, 

 100 fathoms) of one of the main surface-folds of the earth, rising from the 

 3 mile deep of the Atlantic Ocean, which may be visualized as expressing 

 oscillatory effects with the rhythmic progression of the tide-periods of the 

 ebb and flow of the ocean. 6 Taking the Upper Cretaceous as representing 



1 Pocock (1908), The Geology of the Country around Oxford. Memoirs of the Geological 

 Survey, with accompanying Map in colours, I in. scale, and section E. and W. through the City area. 



2 Cf. Islip Railway-cutting. 



3 In great quantity in Iffley Road cuttings, 1922, to 8 in. : smaller forms 3 in. in the Wolvercote 

 brick-pit ; in other cuttings often wholly wanting : remains of timber in clay nodules, corroded and 

 infiltrated with silica and iron pyrites, occur generally in upper Oxford Clay of the Wolvercote pit. 



4 Slopes (1912), Phil. Trans., Petrifications of the Earliest European Angiosperms (Lower 

 Greensand). All such timber from the Oxford Clay, so far examined, was definitely Coniferous, 

 showing uniform tracheides and medullary rays. 



5 Slopes (1910), Annals of Botany, p. 679, Cretovarium. 



6 Such oscillatory effects may be roughly indicated by noting that the present land- level of the 

 Oxford valley is but 200 ft. above the sea, and a drop of a foot in a thousand years would take it to 

 the bottom of a 30-40 fathom sea (comparable with much of the present North Sea area) in about 

 500,000 years. Such a sea at sub-tropical temperature would be full of life, both plant and animal. 



