440 OSBORN AND REEDS PREHISTORY OE MAN IN EUROPE 



In order to be able to express in a convenient manner the synchrony 

 of the banded clay layers of one station with those of another, at a con- 

 siderable distance, the record of each station is thrown into the form of 

 a curve, with the number of annual layers exposed in each section entered 

 on the abscissa and the thickness of each indicated as ordinates. The 

 thick deposits have high points in the curve and the thin layers low ones. 

 By superimposing the curves of each of the stations, one to two kilometers 

 apart, on a line over 800 kilometers long, it was readily noted that a 

 a specific layer could be easily traced a distance of 50 or more kilometers. 



The annual layers during deposition stood in close relation to the border 

 of the land ice. The material of which they consist was carried to the 

 lakes by numerous subglacial rivers which flowed through ice-tunnels. 

 On entering the lake the coarse material was dropjoed in the shape of a 

 delta, while the finer, consisting of sand and clay, was deployed fanwise 

 and deposited at varying distances, the sand first and the fine particles 

 composing the clay still farther out. With increasing distance from the 

 mouths of the subglacial rivers at the ice-borders, each varve (seasonal 

 layer) shows a decrease in its thickness and size of grain. In the sum- 

 mer, when the glacial water was profuse, a great quantity of sand as well 

 as clay reached the lake, while in winter, when there was no melting, or 

 hardly any, only clay was deposited. 



Where the border of the land ice was stationary, one layer was deposited 

 on top of the other; when, however, the ice retreated northward, a portion 

 of the glacial territory became free of ice and a varve was immediately 

 deposited on it and to the southward over previously existing layers. 

 Thus, with the continual retreat of the ice, each subsequent varve trans- 

 gressed toward the north. Its northern boundary indicates the position 

 of the ice-border during its formation. The following up of the northern 

 boundary of each varve permits, therefore, the determination of the 

 gradual retreat of the ice from year to year. It was thus necessary to 

 determine from numerous clay profiles the northern boundary of each 

 varve. This De Geer lias done with the assistance of a number of uni- 

 versity students. 



With the retreat of the ice, the subglacial rivers shifted in a northerly 

 direction, aiid with them the delta of coarse material formed at the 

 border of the ice and the lake. Many deltas in this manner have been 

 extended into a long ridge called an Os. 



The retreat of the ice was accomplished in the summer; in the winter, 

 however, there was a stoppage or short advance. During these advances, 

 in some cases low, narrow ridges of glacial material, sometimes not even 

 a meter high, were formed as winter moraines. The above-described 



