366 PLEISTOCENE GEOLOGY OF THE THAMES VALLEY. 



tracing the beds to the south across their outcrop this northerly 

 dip gradually ceases and the strata become horizontal. Soon 

 after the Thames is reached, however, the beds become again 

 inclined, but this time to the south, so that in the quarries at 

 Greenhithe in Kent the Chalk is seen dipping in this direction 

 at an angle of from 2" to 3" (Fig. 5, p. 367). 



Disturbance of a similar, if not the same, date has resulted 

 in faulting the Cretaceous and Tertiary rocks a little up the 

 Thames Valley from the Grays district. One of these faults has 

 been traced as far east as Erith where it is lost under the 

 Alluvium. The tension resulting from the crumpling has not 

 been great enough to produce faults in the district under con- 

 sideration so far as is known at present, but it has been great 

 enough to open fissures in the Chalk. These fissures are most 

 abundant in that portion of the Chalk which forms the crest of 

 the little anticline, the tension being naturally greatest there, 

 and a good instance of their development in this part of the 

 district was mentioned by Mr. F. C. J. Spurrell in the Report of 

 an Excursion of the Geologists' Association to Grays (64) 



The next point to be considered does not at first sight have 

 much connection with the subject under discussion, namely, 

 the principle governing the formation of terraces. Assume 

 that we have a country whose elevation above the sea may be 

 represented by the symbol A, and that the rivers have adjusted 

 their courses to the base level of our suppositious region, i.e, 

 they are graded. It is likely that the rivers are depositing 

 sediments at this stage ; if slight subsidence of the area goes on 

 they will certainly do so. Suppose that the country has its 

 height above the sea A augmented by a further rise of 50 feet. 

 What happens ? The rivers at once cut through their old beds 

 and commence to excavate the valley 50 feet deeper with the 

 object of keeping their channels graded. The elevation of the 

 land and the erosion of the valleys by the rivers goes on 

 simultaneously. The ultimate result of this is that the deposits 

 laid down by the river when the land was at A, form, when the 

 succeeding period of elevation has ceased, a " terrace " 50 feet 

 above the surface of the river. This is the only way in which 

 river terraces could be formed at the present day, and there is 

 certainly nothing illogical in applying the same principle to the 

 terraces and raised beaches of Pleistocene age. The average 



