78 PHYSICAL GEOGTiAPHY'. 



again removed by the causes before mentioned, but once 

 dropped at the shore they remain there. 



Thus we see that the same natural force that brought the 

 boulders down from their northern home, also placed them 

 in the embankments of those lakes; namely, the expansive 

 power of ice. 



These embankments are really very interesting natural 

 objects, and it is not strange that they have attracted much 

 attention ; but with a correct knowledge of natural forces it is 

 difficult to understand how any person could suppose that 

 human hands had anything to do with their construction. 

 There is certainly nothing in the arrangement of the materials 

 that indicates such an origin; and the liveliest imagination 

 refuses to suggest any object for which human beings could 

 have desired them, or to point out any evidence of human 

 intelligence in their location and plan. 



10. SINKHOLES AND SPRINGS. 



Among the minor features that are found to mark the 

 surface at some limited localities in the State, and which 

 always arrest considerable popular attention, are small 

 circular depressions known as sinkholes. They are almost 

 without exception in the surface of the uplands, and only 

 where these immediately border a valley in the sides of which 

 limestone ledges or cliffs are exposed. 



The mode of their formation is this : the water which falls 

 upon the surface finds its way by percolation down to the 

 underlying strata, and through fissures and interstices in 

 these, out upon the valley-side below, where it issues in the 

 form of springs. A little of the fine surface material is 

 constantly taken into the subterraneous passage by the water 

 which falls upon the surface, and is conveyed by it into the 

 valley in the form of sediment. Upon the surface imme- 

 diately above the point where the percolating water enters 

 the fissure in the rocks, a depression is formed by the caving 

 in of the surface material. Depressions thus commenced 

 deepen by the same cause until the sinkholes are formed, 



