2O Plant-life of tlie Oxford District 



(Shotover is only 560 ft.), could have carried but the scantiest traces of 

 vegetation. 



The River Gravels and Terraces : So long as the land was more or 

 less swept by the ice-sheet, the distribution of gravel was effected by glacial 

 drift ; but as soon as the ice melted, the Greensand and Corallian formations 

 were again exposed to denudation, with the other beds of sand and stone 

 forming the caps of the higher ground. The river-torrents, loosening the 

 entire hill-sides, and fed by melting ice, brought down immense quantities 

 of gravel of characteristic small limestone pebbles, stained yellow-brown 

 with ferric oxides, and with only a small admixture of the older drift, as 

 a post-glacial deposit. The streams flowed apparently at much the same 

 low gradient as at present, with recurrent annual floods, and they swept the 

 mass of the material down the Thames Valley to the sea ; but they left 

 gravel deposits or banks at the flood-levels, especially at points where the 

 main current was checked in flow by the influx of a lateral tributary. 



Interrupted at least at three special horizons, such gravel deposits are 

 distinguished as giving 4 recognizable ' Terraces ', at successive levels on 

 the river-valley margins, separated by periods of more gentle erosion. 

 These latter evidently represent cycles of warmer and colder climates ; the 

 less-marked erosion being the result of a mild winter, with little accumula- 

 tion of snow and ice, and characterized by the animals of a warmer climate, 

 as at the present day ; the gravel terraces themselves being the indication of 

 renewed cold and an intensified flood-system at the melting of the snow in 

 the Cotswolds, and presenting indications of sub-arctic animals. 



The Highest Terrace (' Fourth ') T is 70-100 ft. above the present level 

 of the river, as well as 50 ft. above the next below. Between Kennington 

 and Radley there is a deposit 80-90 ft. above the river with abundance of 

 oolitic pebbles. A similar terrace at Long Hanborough is 90 ft. above the 

 Evenlode. 



The ' Third ' Terrace, formed after a long period of gentle erosion had 

 cut the valley down another 50 ft., is left only as traces at Wolvercote, at 

 40-50 ft. above the river, at a level 30 ft. higher than Carfax. Fossil 

 remains from this Quaternary bed indicate a cold climate with a peat bed? 

 some Alpine plants (Mosses) not known in the district, together with some 

 of the present flora, as also bones of Mammoth, Horse, and Reindeer, 

 together with fine flint implements ; the last being the first traces of human 

 activity in the neighbourhood, and showing that man returned with the 

 larger animals. 



The ' Second ' Terrace, some 20-30 ft. above the level of the river, is 

 largely constituted by the great gravel-bed on which the city was built, 

 extending from Carfax along the Banbury Road for about 3 miles, as not so 

 much a ' terrace ' as a broad pebble-ridge, now residual between the present 

 bed of the Isis and that of the Cherwell, and ranging in some parts to 30 ft. 

 in_depth, commonly 12-20 ft. Bones of Mammoth, Woolly Rhinoceros, 

 Hippopotamus, Horse, Lion, Wolf, Pig, and Reindeer have been recorded. 

 The gravel may be 9-12 ft. deep in the centre of the town, with more 

 admixture of clay than in North Oxford ; the base of this gravel is about 



1 The numbering of these Terraces, following Pocock, loc. cit. , p. 90, might with great advantage 

 be reversed in sequence, the highest being so obviously the ' first ' in order of time, however much it 

 may be the ' fourth ', and also the least definite, in going up from the river. 



2 A. M.Bell (igo^.Q. J. Geolog. Soc., 60, pp. 120-132. A certain amount of error attaches to 

 such determinations where the material is not collected personally with due precautions. Recogni- 

 tion of actual species from mere fragments of seeds and fruits is often open to suspicion, though the 

 genus may be correct. Wind-borne seeds from adjacent weed-vegetation are readily drifted into 

 cuttings, as well as material brought on muddy boots. Mr. Bell's material was looked over at the 

 Botanic Dept. as mere slides of fragmentary debris. One of the likeliest-looking collections of seeds 

 was planted in the garden and came up Chtnopodium album. 



