40 THE THAMES AND ITS TRIBUTARY RIVERS. CHAP. 



marsh, and innumerable inundations may nave spread a level 

 sediment over the area. These effects may have grown less and 

 less by the wearing down of the obstacles on the channel, and by 

 what appears to be certain, a great reduction of rainfall since the 

 immediately post-glacial times. 



Port Meadow, and other apparently flat surfaces on the course 

 of the Thames, suggest the idea of lakes formerly existing in these 

 situations j it is very likely that such may have existed, and been 

 drained by the wearing away of natural dams, as for example at 

 Iffley and Sandford below Oxford. As the dam was lowered, the 

 marginal parts of the old lake would first be uncovered, and the 

 other parts in succession ; and all would be smoothed and levelled 

 by deposits from inundations. The surfaces would be for a long 

 time marshy, full of aquatic plants and shells of lacustrine type. 

 This is very observable in Port Meadow, wherever drains are cut 

 to a moderate depth. In these the gravelly uneven bed of old 

 lake or river channels is met with. 



The Ock. At Abingdon, the Ock, a not inconsiderable stream, 

 brings water from White Horse Hill and the vicinity of Faringdon 

 and Wantage. It occupies the broad Vale of White Horse, prin- 

 cipally excavated in Kimmeridge clay, with detached hills of 

 Portland stone and sands. 



The Thame. The flexuous Thames receives some other additions, 

 running by Clifton Hampden, where the lower greensand makes 

 bold cliffs of conglomerate, to Dorchester, when the Thame unites 

 itself to the now great river ; its spring-heads being far off in the 

 lower chalk of Tring and Prince's Risborough and the oolitic rocks 

 of Quainton and Brill. 



At Dorchester was seated the first Bishop of Wessex; at an 

 earlier date was founded the Roman camp (Durocina), on or near 

 the site of a more ancient British settlement, allured to the pleasant 

 water side, and defended by the mounds on Wittenham Hills. 

 Dwrvb(>p = water, in the Roman name assures us of its having 

 been previously noted by the Britons. 



From this point by Wallingford, Goring, and Pangbourn to 

 Reading, the course of the Thames is a scene of quiet pastoral 

 beauty a green valley winding among softly-swelling mounds of 

 chalk. But it is not less full of painful memories of disastrous 

 civil war. Who can tread the rich fields of Chalgrove, or muse 



