February, 1931 



EVOLUTION 



Page eleven 



CALENDAR 



By ALLAN BROMS 



Fortunately, the layers are not all alike from year 

 to year. Long, hot summers resulted in thick gravel 

 layers, while shifts in outflow caused new kinds and 

 colors of outwash deposits. Such distinctive layers 

 can often be identified over wide areas in several clay 

 deposits. By going from one to another and making 

 careful comparisons, a complete series may be worked 

 out from the overlapping local records to make a 

 continuous calendar for thousands of years. In 

 northern Sweden, fortunately, it was possible to trace 

 the series to the very fronts of existing snow-fields 

 (where just such clay layers are now being laid down 

 each year), so connected definitely with our times. 



Obviously, no layer could be formed at any point 

 until the ice had melted away and uncovered it. As the 

 ice edge receded, each sucfeeding annual layer would 

 extend a bit farther north, overlapping somewhat 

 like the shingles on a roof. The margin of the 

 layer (where it touches bedrock at any date), there- 

 fore marks the edge of the ice-sheet at that date. 

 By this method, the stages of retreat of the Baltic 

 Ice Sheet have been carefully dated as shown on the 

 small map. The entire record, though it does not 

 go back to the very beginning of the glacial recession, 



)d of recording their thick- 

 im Antevs. 



that last covered nearly 

 1 United States, 

 which caused the last Ice 

 ge of the ice receded by 

 ean that the ice moved 



it melted away faster 

 ere and there along this 

 lakes of icy water would 



by earth ridges, on the 

 Abaters, derived from the 

 ly with silts and gravels 

 jlverized both within and 

 g mass of mile-thick ice. 

 vers, when melting was 

 vaters would carry away 

 nces, but let th^ coarser 

 lake bottoms. But when 



the melting, the flow of 

 ) permit the finer silts to 

 o foifti a denser layer of 

 ;at would thus be marked 

 avel and clay layers, each 

 :her and deposition cycle, 

 la "varve." 



RETREAT OF THE LAST ICE SHEET 

 IN NORTHERN EUROPE 



POSITION OF RtTREATiNG ICtEOOE 

 " " " " " Gothi-glacial 



ist cenain years in relslion 10 

 an arbirranljr chosan lero 

 (ont in Sweden nnd a differenr 

 one in Finland) 



MORAINES 



Moraines in general 



. . . JVounger moraines in 

 ■\0enm8rk and Seanla 



-..._ fini-jlacial icedivii 



— poal-glaciat ■■ " 



Present Bainc drainagedividi 



The principal moraines are indicated as follows: F. — 

 Flaming. B. — Bi indcnburg. Poz, — Poznan. Pom. — Pom- 

 merania. — From Antevs. 



The dates of the principal stages of retreat 

 of the Baltic Ice Sheet. 



does go back some 13,500 years, long before the Fun- 

 damentalists' year of Creation. 



A long, oscillating halt of the ice front in the 

 Danish Islands and southern Sweden ended some 

 13,500 years ago. By this time or soon after, our 

 own ice sheet was just retiring from the Great Lakes 

 region. A notion of the succeeding stages in both 

 areas can be gained from the two large maps that 

 show what we have learned from the Great Clay 

 Calendars. The relative completeness of the Baltic 

 Calendar is apparent. The American Calendar has 

 serveral gaps, which we must fill as best we may from 

 other calendar data, such as the gorge-cutting by 

 Niagara Falls, the changes in the Great Lakes, and 

 by tracing across the country the long moraines of 

 dumped earth and rock which piled up along the edge 

 of the melting ice sheet at its greatest extension or 

 where it halted or readvanced slightly in its general 

 northward retreat. Such moraines are indicated by 

 dotted lines on the maps and do serve to mark con- 

 temporaneous locations of the ice front, since they 

 were deposited at much the same time. The year 

 figures given on the geologists' working maps are not 

 oar own calendar dates, but are relative to arbitrary 

 base locations of the ice front (for no actual dates are 

 ever known at the outset). But on the American 

 map, figures have been guessed at for the number of 

 years after the beginning of glacial retreat. Both the 

 southern and northern ends of this record are miss- 

 ing, but good guesses seem to be that this beginning 

 occurred about 40,000 years ago, certainly more than 

 30,000, probably less than 50,000. In this brief 

 period (brief geologically) the great ice sheets have 

 shrunk to some mountain glaciers in the Alps and 

 Scandinavian mountains and to a more considerable 

 ice cap over Greenland. But even the Greenland ice 

 cap, thick and immense though it be, is a mere rem- 

 nant of the miles-thick ice sheet which once covered 

 some four million square miles of our continent. 



References: Ernst Antevs, "Recession of Last Ice Sheet in 

 New England" (1922); "The Last Glaciation" (1928). 



