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EVOLUTION 



June, 1931 



New York in the Ice Age 



By ALLAN BROMS 



IN the Bronx Zoo stands the famous "Rocking Stone," a 

 thirty-ton boulder perched in such nice balance that a fifty 

 pound pressure can rock it two inches on its narrow base. It 

 is a typical ice-borne "glacial erratic" brought and left here 

 by the great ice sheet of the Glacial Period. LJnder it, the 

 bedrock of Manhattan Schist is planed level by the overriding 

 ice and a nearby rock knoll to the northwest has several clear 

 grooves, one or more of which may well have been cut by this 

 very boulder as the ice sheet, into which it was frozen, scraped 

 it over the surface. 



Elsewhere in New York are many exposures of this and 

 other bedrocks similarly planed and scratched, and almost 

 anywhere one can see such erratic boulders brought by the 

 invading ice sheet. Many of these turn out to be derived from 

 the Palisades across the Hudson River, indicating that the ice 

 moved, not down the river valley, but southeasterly across it, 

 a fact confirmed by the direction of the glacial scratches or 

 striae on the bedrocks. Other erratics in the vicinity are known 

 to have come from the Catskills and points even farther north. 



These and other ice signs are to be found all over New 

 England, Canada, the states bordering the Great Lakes and 

 all northwestern Europe. This fact was first discovered by 

 Louis Agassiz, who concluded that all these areas, totaling some 

 six million square miles, were at one time covered by thick ice 

 sheets, ever expanding from northern centers of snow accumu- 

 lation. Now that we have learned to read them, these ice 

 signs tell an eloquent story of repeated glacial invasions, all 

 the more interesting to the amateur because they are unmis- 

 takable and furnish the essential clues to most of our surface 

 geology. 



Often these erratics are associated with a confused mixture 

 of smaller rocks, gravel, sand and clay, all heaped together in 

 the usual disorder of the glacial moraine, wherever the melting 

 ice happened to dump them. Here and there, the waters of 

 melting may have done some sorting of the gravel into irregu- 

 lar cross-bedded layers within the morainic confusion and 

 spread the sands out into level outwash plains before the 

 front of the ice. Long Island provides the perfect example. 

 Its north half consists of terminal and retreatal moraines 

 stretching its whole length, while the south half consists of 

 flat outwash plains. 



Because the outflowing waters were unable to transport 



heavy boulders, these outwash plains have no erratics whatever. 

 But in the morainic hill belt they are everywhere, though 

 usually buried in the earthy rubbish heap which we call glacial 

 "drift or till." These glacial dumps are most irregular in 

 form, being piled high into a hill here, being lacking so as to 

 leave an isolated pond there. This was partly due to irregular 

 melting, but also to uneven loading within and upon the glacier. 



Moraines, showing where the ice sheet halted. 



"Rocking Stone," New York Zoological Park, an ice-ttansported 

 boulder on a glaciated surface. 



For as the ice slowly crept southward, it had frozen on to such 

 underlying rocks as it could loosen, carrying them along, 

 dragging them along its bottom, grinding them together within 

 its mass. But sooner or later it faced the onslaughts of the 

 hot sun, turned to water, dropped everything, and ran — as 

 water. Sometimes a great block of ice would be left buried 

 in the moraine heap, quite deserted by the retreating ice front. 

 When eventually the ice block melted, it left a deep hole in 

 the hilly landscape, a typical "kettle hole." 



Because they are usually buried, erratic boulders are not so 

 conspicuous in the terminal moraine as they are over areas 

 scoured bare by the overriding ice sheet and then covered but 

 thinly with a scanty moraine left as the ice front rapidly 

 retreated. In such areas there would also be a generous flow 

 of waters from melting, tending to wash away the finer glacial 

 materials, not overly abundant at best. Consequently, the really 

 fewer erratics now stand out conspicuously in contrast to the 

 nearly barren bedrocks. LJp New England way they build 

 fences of them, which also gets them out of the way of their 

 plows. 



Many bedrock masses have been smoothed and carved by 

 the overriding ice, being left as low elongated knobs called 

 "roche moutonnees," (rosh mootone), a name best remembered 

 by its French derivation, roche meaning rock, moutonnees 

 meaning mutton, because they looked like sheep's backs. Their 

 length is in the direction of the ice movement, the end towards 

 the oncoming ice normally rising with a smooth upward slope. 



