Correspondence — A. R. Hunt. 619'/ 



valleys to make watertight barriers in the construction of reservoirs 

 revealed the fact that the bottom of the valleys, wherever it was 

 formed of shales and thin sandstones, was more or less folded and 

 contorted. These folds and contortions caused the shales to let th& 

 water through with more or less freedom, and he had been called 

 in repeatedly to advise as to how far it was necessary to carry the 

 puddle trenches down below the valley bottom. He found, as, 

 a matter of experience, that these folds were superficial, and if the 

 sinking was made to a sufficient depth below the bottom of the 

 valley they disappeared altogether. It was therefore obvious that 

 they were not due to deep-seated movements of compression resulting; 

 from the contraction of the earth. They are due to the relaxation 

 of pressure caused by the removal of the rock by denudation from, 

 the area of the valley, and are analogous in every particular ta 

 the • creep ' in coal workings, caused by the excavation of coal,, 

 by which the surrounding strata crush down into the area of 

 relaxed pressure and ultimately fill it up. This may be studied 

 in any coal-pit whex*e there is a superincumbent pressure, say, of 

 more than 1,000 feet. 



Two illustrations of folding and faulting by relaxation of pressure 

 are presented by the puddle trench of the Langsett reservoir belonging, 

 to the Sheffield Corporation, and by the two reservoirs now under 

 construction on the head waters of the Derwent by the Derwent 

 Water Board. In the first of these the foldings in question at; 

 the bottom of the valley in the shale under the first grit are strongly 

 marked at the surface. These folds gradually disappear, and are 

 based upon a hard black unmoved shale oifering a good foundation 

 about 60 feet below the bottom of the valley. This is in the valley 

 of the Little Don. The thickness of rock removed from the bottom' 

 of the valley amounted to no less than something like 8,000 feet 

 of Coal-measures and Millstone Grit. In the case of the Derwent, 

 in which the folding is much more marked and is accompanied by 

 faulting, the thickness of rock removed amounted to at least 9,700 

 feet (7,200 feet of Coal-measures, 2,000 feet of Millstone Grit, and 

 at least 500 feet of Yoredale). In this the movement had not 

 extended beyond a depth of 90 feet. In the case of the Derwent 

 reservoir lower down the river there are two systems of folding and 

 faulting which do not penetrate beyond 60 feet from the surface. 

 At that point a good foundation is found for the puddle trench 

 of the embankment. These points are of considerable importance 

 in considering the sites for reservoirs. — From Water, November 15th, 

 1904, No. 71, p. 491. 



COZRIRESI^OliTID'^lsrCS- 



THE NEW QUESTION OF RIPPLE-MARK. 



Sir, — In the August number of the Geological Magazine 



I proposed to offer at some future time an explanatory paper on 



the formation of ripple-mark ; writing then under the impression 



that experts were quite agreed upon a subject not generally known. 



