128 



to shoot. Like the sandstones, also, it will not endure frost action and 

 insolation. Structurally and texturally it is very imiiermeable, and ideally 

 free from objectionable joints and crevices. Sound rock will usually 

 be found fairly near the surface, especially on steep slopes. 



This shale formation is known to geolcgists as the New Providence 

 shale. It is 400 to 5lX> feet thick. Where, in the eastern portion of the 

 Knobstone area the larger streams cut through the New Providence 

 shale, they enter the ujiper portion of the New Albany black shale, 

 which is, like the former, a very Impervious format inn. It is evident, 

 therefore, that any part of the Knobstone region will afford satisfactory 

 foundations for dams of all sorts. 



The proper type of dam for the Knobsttme region will depend, of 

 course, (in the conditicms at the particular site. In the majority of 

 cases comparatively narrow, deep. steei>sided valleys will have to be 

 dealt with; and this will be so iu practically every instance where only a 

 few square miles of catchment are needed.^ For this type of valley 

 where the breadth of the valley floor is not more than 300 feet, the 

 most satisfactory, as well as the cheapest type of dam, is the concrete 

 dam, arched up-stream to a radius of .SOO feet or more. Such a dam, 

 depending to a large degree on its curvature for its stability under water 

 pressure, may be built with less material than any other type of safe, 

 permanent dam. The construction should be such that the water face 

 of the dam is perfectly tight. The balance of the dam may, however, 

 be built of rubble concrete (uncoursed stone) i. e., large stone imbedded 

 in a mortar of concrete. Some reinforcing steel to assist the sti*ucture 

 in taking up the strains due to setting of the concrete and to thermal 

 readjustments, will tend to prevent cracking. After the pressure of the 

 water comes against the dam, there should be no tendency of an arch 

 dam to crack. The ends and base of the dam should be mortised into 

 the solid, unweathered rock, and every precaution should be observed 

 to make these contacts perfectly water-tight. 



'From the rainfall and run-off data given above, it will be seen that it is not 

 gate in the present region to allow more than 2.1 per cent, of the rainfall of a dr.v 

 season as available for impounding. This will approximate .300,000 gallons per 

 day from each square mile of catchment with reservoir capacity sufficient to hold 

 the entire i-un-off of the year. With reservoir capacity sufficient to hold the run- 

 off of the three driest years — it is not economical to increase capacity beyond this 

 point — the yield can be increased by about 50 per cent. A very full discussion of 

 this subject will be found in the article on Water Supply in the 11th edition of the 

 Encyclopaedia Britannica, by Mr. G. F. Deacon. This article is a mine of infor- 

 mation on most phases oi water supply. 



