Dr. A. Irving — The Thames High-level Plateau Gravels. 499 



of the Chalk extension to tlie north-west, across which these rivers 

 flowed, had been already so deeply furrowed by them that they had 

 even then begun to saw down into the Jurassic rocks. Eolled 

 fragments occur of basic igneous rocks and other crystallines, the 

 origin of which it is not easy to determine, though they may have 

 come from Charnwood or from the igneous intrusions of Warwickshire 

 and Worcestershire. Sarsens and flinty detritus are common enough 

 in the gravels as well as Tertiary flint pebbles. 



Not only are these gravels much older than the Boulder-clay 

 {and its equivalents), superimposed upon them, but their relation 

 to the present river-drainage shows that they are older than even 

 the pre-GIacial valleys of erosion in the Chalk, now buried beneath 

 alluvial detritus, which has been proved by a well in the valley of 

 the Stort to a depth of 170 feet ; while on the watershed between 

 the Stort and the Cam (also a buried valley) a well-section 

 close to Elsenham Station has proved a depth of 90 feet of drift 

 before the Chalk is reached. We do not know that we have there 

 reached the lowest line of the submerged valley, the original Thalvoeg. 



The author regards these plateau gravels as the deposits of 

 Mercian rivers, which flowed through the gaps in the present 

 Chalk range, such as those of Elsenham and Hitchin, towards the 

 ancient arterial Tamisian line of drainage of southern England 

 during the great Miocene elevation of north-western Europe, long 

 before the present escarpment of the Chalk was formed, and there- 

 fore before the initiation of the Mercian river-system, as it exists 

 to-day with its convergence towards the Wash and the Humber. 

 In one section at Stansted a fault of five feet throw cuts through 

 the Chalk, the Reading Beds, and the stratified gravels, showing 

 their common participation in later earth-movements, while the 

 Glacial deposits above are unaffected. The author conceives these 

 well-stratified and indurated gravels,^ splendid examples of which 

 may be seen at Thorley and by the Hallingbury road on the opposite 

 site of the Stort, to be the deposits of a river, at a time when the 

 now buried valleys of the Stort and the Cam in the initiatory stages 

 of their erosion formed perhaps one continuous channel, and before 

 the axial movement took place, which has had much to do with 

 differentiating the present Tamisian and Mercian systems of river- 

 drainage. That movement probably dates from the close of the 

 Miocene period, the corresponding subsidences to the north-west 

 and south-east of the axis of anticlinal flexure being indicated by 

 the ingress of the sea and the consequent Crag deposits in East 

 Anglia on the one side, and by the northerly dip of the strata 

 (with perhaps a true dip to the north-west) as seen in the cliffs 

 at Hunstanton on the other side. This latter subsidence, together 

 with the recession of the Chalk escarpment by denudation, 

 determined probably the present system of drainage towards the 

 Wash in Pliocene times. The nature of the detritus which has 

 buried the ancient Stort Valley is exhibited at the present time in 

 two open gravel-pit sections at Stansted on a magnificent scale, one 

 on either side of the valley, the section in the largest pit showing 



