1 6 Plant-life of the Oxford District 



larly fine at this period, expressing massive reef-formation (cf. Honey-comb 

 Coral Isastraea of Cumnor and Headington). Limestones may be so hard 

 as to be utilized for building-stone in numerous local quarries (Headington 

 Quarry), as the inferior building-material of the district. Such beds extend 

 to a thickness of 12-40 ft., and are characteristic for Headington. Lime- 

 stone rocks, cut down to Calcareous Grit sands, are still worked at Heading- 

 ton, and Coral Rag land, with abundance of broken corals, is seen in the 

 large arable fields of Cumnor, N. Hinksey, Wytham Hill, Headington, 

 Elsfield, Beckleyand Kennington, as also on the western slopes of Boar's Hill. 

 The upper limit of the Coral Rag gives, again, sands of more inshore 

 deposits, and the upper surface of the stone may be water-worn, indicating 

 exposure as land before it sank again to repeat a clay deposit of the 

 Kimeridge horizon. Estuarine conditions were probably never far away ; 

 clays with nodules appear in the Upper Corallian of Littlemore, and the 

 Ampthill clay which comes to the surface 7-8 miles to the east, at Waterperry 

 and Worminghall, belongs to the same period. The most characteristic 

 exposure is that of the railway-cutting at Littlemore, showing successive 

 strata from clay with nodules above to a basal layer of hard rock covering 

 the sandy mass of Calcareous Grit. 1 



The following Kimeridge Clay expresses again the silty estuarine 

 deposit of a denuded area of volcanic activity ; iron and sulphur in quantity 

 being mixed with the lime debris of the sea. The dark clay is coloured 

 with ferrous sulphide and carbonaceous matter which may indicate vegetable 

 debris and algae drifted with the slow current. As in the case of the Oxford 

 Clay, the deposit is that of a shallow sea full of life, including great saurians 

 (Pleisosaurus, Ichthyosaurus) and common shells (Cardium, Pec ten, Pinna) 

 of recent seas, as well as large Ammonites (Chawley cutting) and oyster 

 shells (Ostraea deltoidea, Headington), but no longer Gryphaeas. At 

 Chawley Brickworks strata indicate masses of mud with debris of empty light 

 Ammonite shells, as if floated to the limit of the tide-mark, suggesting 

 estuarine beach-levels. At Shotover this clay is estimated at 100 ft. thick, 

 with again traces of lignite. The most characteristic exposure is that of the 

 cutting at Chawley, which is encroaching on Cumnor Hurst. This clay 

 forms the greater part of the soil of Bagley Wood, as well as broad areas 

 under Shotover, and to the south of Garsington Hill. 



Above the Kimeridge Clay the conditions again conspicuously change to 

 sandbank formation with clean water, giving ' Greensand ' with glauconite 

 (but now red with iron), and white coral limestone. The lower beds, about 

 60 ft. thick, are again separated from the upper 40-50 ft. by narrow clay 

 bands. At Shotover the whole may be 100 ft. thick. These imply a long 

 period of clean sea-water and coral-reef; though some may have been since 

 metamorphosed by permeation of silica and iron. 



Together with these, other traces of Cretaceous deposits also occur as 

 Portland Beds, capping the more hilly districts to the SE. (Baldon, Milton) ; 

 and though these may be insignificant in floristic value, they remain the 

 only guide to the progression of the formations and the condition of the land 

 throughout the rest of the Cretaceous epoch well into the Tertiary. Traces 

 of Purbeck beds, following on after the Portland beds at Shotover, are seen 

 at Wheatley and Garsington ; the special interest of which centres in the 

 fact that they are more distinctly estuarine, grading into definitely fresh- 

 water deposits with fresh-water shells (Planorbis). In these Garsington 

 deposits fragments of Coniferous wood occur, indicating the debris of forest- 

 land ; while traces of an actual land- surface soil of this period have been 

 noted 10 miles away to the NE. at Brill. 



1 Pocock (1908), loc. cit., p. 35, for detailed description. 



