141 



it with the limestone retrioii. Smaller amimiits of water enter the ^'roniul 

 tlian in tlie limestone area, howevei'. (iwin.i,' to the steepness of the slopes, 

 especially wliere these are not under forest. The mn-oft: is concentrated 

 more into the winter and spring months, flood stages are higlier, and the 

 streams carry a greater amount of sediment than is the case in the lime- 

 stone region. 



Furthermore, there are. in contrast with the limestone region, beds 

 of impervious shale in the Chester formation, that where favorably located 

 miglit serve as foundations for dams. It should be noted, however, in tliis 

 connection that these shales are less Ann than those of the Knobstone 

 formation, and would consefjuently be less capable of sustaining the weight 

 of heavy structures.^ Where the shales and sandstones are underlain by 

 thick beds of limestone, they will often be found to have collapsed into 

 large solution cavities in the latter, and consequently to present a con- 

 fused and broken structure, wholly unfit for the foundation of a dam. 

 Numerous examples of this collapse may be seen in the cuts on the Illinois 

 Central Railroad near Stanford, indiana.- 



Recause of their greater permeability the formations of this area will 

 also be found to be weathered to a greater depth than those of the Knob- 

 stone area. In view of all these facts it will be seen that great care should 

 be exercised, and very careful study of the geological conditions should be 

 undertaken in every individual case before placing any impounding struc- 

 tures upon the rocks of the Chester formation. The Mansfield formation, 

 while not as leaky as the limestones of the driftless area, is nevertheless 

 not a favorable formation for impounding water, on account of its porosity. 



In regard to deep wells, the Chester-Mansfield area is similar to the 

 limestone region to the east. Deep wells ordinarily produce mineral water. 

 In the western edge of the area, where the sandstones of the Chester or 

 Mansfield are deeply buried, it may be possible to obtain from them water 

 not too highly impregnated with mineral matter for domestic use. The 

 writer is not in a position to speak with authority on this point. It is 

 quite likely that the upward moving water from the limestones beneatli 

 would even here cause enough admixture to render the water of the sand 

 stones unfit for use. 



1 The reccut faiUire of tho Austhi dam hi Ponnsylvauia, was due to the pres- 

 ence in the substratum iipon which the dam stood, of a bed of soft, slippery shale. 

 The dam seems to have slid bodily forward, carrying the rock on which it stood 

 with it. Even in this case, however, it is very doubtful if there would have been 

 any failure had the dam been arclied, as a dam of its length should have been. 



^ See Greene, loc. cit. 



