312 



NOTES ON A MAP, ETC. 



to its present grade above the sea. While falling thus, the opposing outcrops of the 

 upper rocks retired from one another, eastward and westward, and let the Blue Grass 

 area between them widen slowly to its present size. And still the work goes bravely on. 

 The Mammoth Cave is only one of many thousands which border the central anticlinal 

 Blue Grass country, crossed by the Ohio Biver. 



The same method of erosion, by caverns and sink-holes, is observable in all the Appa- 

 lachian limestone valleys east of the coal area, explaining, I think, the depth of their 

 limestone floors, as compared with the sandstone mountains which environ them. 



There can no longer be entertained a doubt of the existence of vast sheets of ice upon 

 the surface of this continent at certain epochs. Even changes of sea-level may be calcu- 

 lated by considering what must have been the amount of water necessarily abstracted from 

 the sea at first for their production, and again restored to it at their destruction. But 

 we surely cannot accord even to the heaviest conceivable ice-cake of the glacial or any 

 other era the honor of eroding our topography. Without referring to such profound 

 abysses as the Valley of the kSaginay in Labrador, the Yosemite Valley in California, or 

 the new-found chasm of the Albert Nyanza in Tropical Africa, it is sufficient to look at 

 the map before us to feel the absurdity of the suggestion. 



If such an ice-cake ever passed over oxir mountains, it must have had its southernmost 

 limit somewhere along a line drawn through the middle of Pennsylvania. Otherwise, 

 why has no diluvial scratch or groove ever been put on record from any point south of the 

 Turnpike notch in the summit of the Fourth or Peters Mountain, fifteen miles north of 

 Harrisburg? And why do not genuine erratics exist in Southern Ohio? Outcrops in 

 Virginia could receive and should retain striae quite as well as outcrops in Pennsylvania, for 

 they are in all respects similar. Ice had no hand in Appalachian erosion. All it has done 

 has been done in the north, and its only action was to polish a surface already made. It 

 has left no other trace of its existence. It had no mountain summits from which to 

 receive moraine matter, and no moraine has been as yet discovered out of New England 

 and Northern New York. The northern outcrop-sides of some of our east and west-run- 

 ning (eight to twelve hundred feet high) Appalachian ridges in Pennsylvania, arc as 

 uniformly sheeted with angular blocks, detached by frost and tumbled down by slow 

 wastage of supporting shales, as are the southern outcrop-sides of the rest of them. Ice 

 has had no hand in this, except the ice of winters innumerable, that of 18(5(5 included in 

 the count. These angular-stone facings are no moraines ; and the solitary alleged 

 moraine, at the mouth of one of the twig vales of a branch of Pine Creek in Northern 

 Pennsylvania, ought to be well studied before it be accepted as a product of anything else 

 than the freshets which carry destruction periodically down along our river-beds. 



It has not been my intention to write a memoir on Erosion, but to show how far-reach- 

 ing those suggestions are which such a map produces to the eye of the geologist. 



