368 PLEISTOCENE GEOI.OGV OF THE THAMES VAl.I EY. 



height of the High Terrace is about 100 feet higher than that 

 of the river of to-day, a relation which is exhibited from the 

 eastern portion of Essex up the valley as far as Reading if not 

 further. Very commonly also we have a Raised Beach at the 

 same relative level occurring on different parts of the coast 

 which we would correlate with the High Terrace. 



It follows from this that in High Terrace times the land 

 stood at an elevation 100 feet lower in relation to the level of the 

 sea than at the present time. This being the case it means a 

 corresponding general rise of the saturation plane in High 

 Terrace times since it is evident that strata now 100 feet above 

 that plane were then just beneath it. 



Prestwich objected to the view that fissures favourably 

 influenced the dissolution of the Chalk on the grovmd that acidic 

 water would pass so freely through them that very little solvent 

 action would be carried on upon their sides and edges. But this 

 difficulty is removed when we consider the physical conditions of 

 the High Terrace stage in the light of recent scientific advance. It 

 is now more than probable that the view expressed above as to the 

 formation of terraces is correct and this being so it is clear that 

 the fissures in the Chalk of Grays were in High Terrace times 

 completely immersed below the saturation plane. It is evident 

 that such fissures, granted favourable conditions, will always 

 form the vulnerable portions of a rock which will most readily 

 yield to denuding agents whether they be of a mechanical or a 

 chemical nature. They constitute, together with joints, natural 

 lines of weakness and greatly assist in the disintegration of 

 rock-masses. Mr. Rutley's experiments in the dissolution of 

 blocks of chalk with weak acids, described by him in his 

 paper on "The Dwindling and Disappearance of Limestones " 

 (66), fully corroborate this view as applied in the present 

 connection. 



The solvent will most readily attack the edges afforded by 

 the junction of the walls of the fissures with the upper surface of 

 the Chalk, because there it will have the greatest superficial area 

 to act upon, while the strains, of which the fissures are resultant, 

 will have increased this large working surface by the partial 

 shattering or splitting of the surrounding rock. The dissolution 

 will, therefore, tend to widen the upper part of the fissure by 

 gradually rounding off the edges in such a manner that a shallow 



