PLEISTOCENK GEOLOGY OF THE THAMES VALl.EY. 365 



acidic water upon the Cliaik of the district. It is evident, 

 however, that a process which in other districts has produced, 

 but comparatively infinitesimal results must have been favoured 

 in this neighbourhood by conditions which have greatly accele- 

 rated and regulated its working. Sir Joseph Prestwich in his 

 masterly essay upon the origin of the Sand and Gravel-pipes in 

 the Chalk (9) was the first to attack the subject of the conditions 

 which must necessarily govern their formation. He maintained 

 that pipes were not only the result of the chemical dissolution of 

 the Chalk, but, moreover, under existing conditions in the great 

 majority of instances they could not now be formed. He argued 

 that the existence of a higher level of saturation was essential 

 to the formation of pipes, and this increase in height was of a 

 secondary nature, depending not so much upon the elevation 

 of the land above the sea as upon the relative permeability of 

 the Chalk and its superincumbent strata together with the 

 unbroken continuity of the latter across broad stretches of 

 country. Further he held that the pipes did not necessarily 

 imply the existence of fissures or joints in the Chalk, but that 

 such occurrences would retard rather than accelerate their 

 formation. In the case before us, however, Prestwich's con- 

 clusions cannot be applied in their totality, but rather require 

 some little modification. 



It is clear that there is one important distinction between 

 the dissolution of the Chalk into pipes on the one hand and the 

 formation of valleys by the same means on the other. In the 

 one case the conditions rendering the Chalk liable to the attacks 

 of solvents are extremely local and may be but slight, but in 

 the case of the valleys the conditions are not local, in the sense 

 of the term applied to the pipes, but on the contrary extend 

 over a considerable horizontal distance, giving rise to definite 

 lines or planes of weakness. The nature of these lines of weak- 

 ness, which constitute the valley axes, is shown by the geotec- 

 tonic geology of the district. 



The Chalk of the Grays Thurrock Area occurs as an inlier 

 brought up by a small anticlinal fold, later in date than the great 

 VVealden and London Basin movements as is indicated by the 

 following data. At Stifford it in common with the Lower 

 London Tertiaries dips to the north at an angle of about 

 lo", and thus is soon lost beneath the London Clay. But in 



