Page ten 



EVOLUTION 



February, 1931 



How annual clay layers were deposited in 

 fresh water off the receding ice front during 

 hree successive years. — From Antevs. 



'T^HERE Still live many who take seriously the chron- 

 -*■ ology drawn about 1650 by James Usher, 

 Archbishop of Armagh, who set the date of Creation 

 in the year 4004 B. C by the simple method of 

 adding Adam's 930 years to Enoch's 365 years to 

 Methuselah's 969 years, etc. Dr. John Lightfoot, 

 Vice-Chancellor of Cambridge University, promptly 

 "improved" on this dating by specifying that the 

 Creation occurred on October 23rd, at nine o'clock in 

 the morning. But later scholars, of equal "author- 

 ity", have placed the date variously at 4710 B. C. and 

 5872 B. C, the resulting debate lasting (in certain 

 circles) to this very day. (See page 19.) 



But meanwhile, for half a century, the geologists 

 have been confidently talking of millions and lately 

 of billions of years since the Earth was born, and 

 have developed dependable methods for closely esti- 

 mating the ages of our oldest rocks. But especially 

 conclusive is a method now used by the glacial geolo- 

 gists to date (almost to the year in some cases) 

 the progressive recession of the great ice sheets 

 from northern Europe and North America, which 

 dates take us back definitely some 13,500 years, long 

 before the orthodox- dates of Creation. 



The method, originally suggested a century ago by 

 an American geologist, Alfred Smith, and first ap- 

 plied about 50 years ago by Baron Gerard DeGeer 

 of Sweden to working out a post-glacial chronology, 

 consisted in counting the number of layers in clay 

 deposits, each layer representing a year, much as we 

 measure the age of a tree by counting its annual 

 growth rings. 



Several years ago, in a rear storeroom of the 

 American Museum of Natural History, I was privi- 

 leged to examine a collection of banded clays care- 

 fully preserved in long metal trays and soaked with 

 glycerine to keep them from drying and shrinking. 

 From them, Dr. Chester A. Reeds and other geolo- 

 gists were reconstructing the history of the New Jer- 

 sey, New York and New England areas since the 

 close of the Great Ice Age, long before the coming 



THE GREAT CLAY 



of the white man. They already had a complete 

 year by year record covering thousands of years, but 

 , not definitely connected with modern dates. 



They had examined hundreds of clay pits (often 

 already nicely dug out for brick yards) along the 

 Passaic, Hackensack, Hudson, Cormecticut and Mer- 

 rimac rivers. They either measured the series of clay 

 layers by marking them on long, strong strips of 

 paper (see center illustration), or they took samples 

 of the clay itself by pressing a long, narrow metal 

 tray vertically against the smoothed clay wall and 

 then cutting the enclosed clay loose from the mass. 

 This latter method was often necessary when the 

 layers were too alike in thickness, but showed distinc- 

 tive differences in color or texture by which they 

 could be identified in several locations. In the lab- 

 oratory, all the measures were charted for ready 

 comparison and the work of counting for actual 

 dating was done. 



At that time, one of the group. Dr. Ernst Antevs, 

 was away up in the wilds of Canada, laboriously 

 gathering more samples and measuring clay layers in 

 an attempt to extend the series of years and fix defi- 

 nite dates for all the various stages of melting and 



RETREAT OF THE LAST ICE SHEET 

 IN NORTHEASTERN NORTH AMERICA 



POSITION OF RETREATING ICEEDGE 



at eerTBin years in relalion To 



an erbilranly chosen zero 



(one intheConnechcut Valley 



and a different one in itie 



Timistiaming-Aljtnbi legion) 



at beginning of KlrWeld stagk 

 stendofLakelroquoiE " 

 ... etbeginningof marine " 

 MORAINES 



general 



The letters indicate moraines: T.M. — terminal moraine. R. 

 — Ronkonkomon, H.H. — Harbor Hill. M. — Mississinawa. 

 D.— Defiance. A.— Alden. N.F.— Niagara Falls. P.H.— 

 Port Huron. — From Antevs. 



Annual clay layers and 

 nesses on a strip of paper 



retreat of the great ice 

 all Canada and the not 



When the long cold s 

 Age came to an end, tl: 

 melting. This does nc 

 backward, but merely 

 than it moved forward 

 retiring ice front, ponds 

 form, banked on the s 

 north by ice itself. Tl 

 melting ice, would be ) 

 that had been ground ai 

 beneath the moving, cri 



During the warm ! 

 rapid, the swiftly flowi 

 the finer silts to great 

 gravels settle at orice on 

 the cold of winters sto) 

 waters would slacken a: 

 settle slowly in still wal 

 greasy clay. Each passi 

 by an alternating pair c 

 pair recording a yearly 

 called (from the Swedi 



