A. J. Jukes-Browne — Solution in Valleij-makhuj. 533 



•show underground action. As regards the ' marked character ' of 

 these Chiltern valleys, I suppose he means their depth and the manner 

 in which they have been cut back into the escarpment-ridge, and 

 sometimes even through it. I cannot see anything in these characters 

 incompatible with the view that these valleys are the modified relics 

 of an ancient system of drainage superimposed upon and finally 

 transferred to the Chalk. 



The high-level gaps in the escarpment are surely the truncated 

 portions of valleys which date from a time when its frontal face lay 

 much further west. The deeply-sunk terminations of the valleys 

 inside the escarpment-ridge are due partly to the solution of the chalk 

 ■along the previously established waterways, and partly to the frequency 

 of landslips when the rainfall was greater than it is at present. 



In this connection I would observe that the part taken by landslips 

 in the formation of these valleys has not been sufficiently considered, 

 and I mvist plead guilty to having omitted any mention of landslips 

 or of solution when describing the formation of valleys in Chalk 

 districts in a recent memoir,^ though they were duly credited with 

 a share of the work in my paper on the ' Clay-with-flints.' ^ 



I believe that small landslips are more frequent in the dry valleys 

 of Chalk districts than is commonly supposed. I have seen many 

 such landslips on slopes of solid chalk where there were no permanent 

 springs and no impervious substratum. The valleys I have in mind 

 ■are the upper parts of those on the dip-slopes of the Chalk escarpment 

 in Wilts and Dorset ; these have very steep sides, the angle of incline 

 being often between 25° and 28°, and small slips from these slopes 

 often take place after heavy rains, leaving bare faces of chalk with 

 a ruckle of soil, rubble, and turf below, the whole forming a con- 

 spicuous scar on the prevailing green turf slope. These slips seldom 

 exceed 5 or 6 yards in width, but they illustrate one of the processes 

 which have operated both in widening the valley and in causing the 

 recession of the valley -head. If such slips occur at the present day, 

 how much more frequent must they have been in earlier Pleistocene 

 time, when the precipitation of rain and snow was so much greater 

 than it is now ? 



Chemical solution must also have been operative in deepening these 

 valleys, but I think it acts from above downward and not from below 

 upward. After a heavy rainfall a pai't of the water falling on the 

 escarpment and on the ridges between the valleys is directed into the 

 valley- ways, where it soon sinks beneath the surface, and having 

 acquired some carbonic acid and some humic acids in its descent of 

 the valley-slopes, it cannot fail to exercise a solvent action on the 

 chalk below the valley-floor, and thus to deepen the valley by the 

 removal of chalk in solution. 



But this solution process is confined to the depth of a few feet from 

 the surface, and is not the process imagined by Mr. Spicer. He 

 expressly says that the subterranean water-flow " will always tend to 

 produce a master-joint, which will carry an underground stream in 



1 " The Cretaceous Eocks of Britaia " : Mem. Geol. Survey, 1904, vol. iii, p. 418. 



2 Quart. Journ. Geol. Soe., 1906, vol. Ixii, p. 158. 



