PLEISTOCENE GEOLOGY OF THE THAMES VALLEY. 369 



groove of considerable width is formed and into this groove the 

 superincumbent and elastic strata slowly descend, and thus a 

 valley is gradually formed which marks the line of fissure. 



It has been previously said that the Chalk inlier of Grays 

 Thurrock really forms part of a small anticline. On the summit 

 of the dome the fissures are probably widest, but they have not 

 given origin to the biggest valleys. The little valley seen to the 

 west of Belmont Castle mentioned above is a case in point and 

 the explanation of this apparent anomaly is not far to seek. 

 From their elevation it is evident that the dissolution of the 

 Chalk and the consequent widening of the fissures occurring 

 there could only have been carried on for a short time after 

 elevation of the land had set in at the close of the High Terrace 

 stage as the upper portions of the fissures would soon be placed 

 above the level of saturation. On the flanks of the little anti- 

 cline, however, the Chalk strata are carried lower, the Tertiaries 

 being consequently in greater thickness. Here the valleys are 

 much larger, as we should expect, seeing that the fissures, of 

 which they form the surface indication, have always been below 

 the plane of saturation since High Terrace times from which 

 date the erosion of the Chalk by acidic water has been continu- 

 ally carried on. (Fig. 6, p. 367). 



There is a question as to the date of the movement bringing 

 up the Chalk at Grays and so forming an anticline, the con- 

 sideration of which we do not wish to enter into in this part of 

 our paper. It will, therefore, suffice to say that the evidence so 

 far as we have examined it at present tends to show that the 

 movement which caused the crumpling of the Chalk and 

 Tertiary strata was also the movement which elevated the land 

 at the close of the High Terrace stage. 



VIL CONCL USIONS. 



The conclusions arrived at in the papers on the Ilford 

 District require a little modification, principally with regard to 

 the High Terrace Drift. In the first of the former papers it 

 was argued that this portion of the Pleistocene period was 

 characterized by a climate of considerable rigour. Undoubtedly 

 there is evidence of the action of river-ice in the deposits of 

 this terrace, but since these deposits have been yielding to us a 

 fauna as rich as that of the brickearths at a certain locality in 



