328 GEOGRAPHICAL DISTRIBUTION OF THE LOESS. CHAP. xvr. 



Geographical Distribution of the Loess. 



It was stated in the last chapter, p. 302, that at the time of 

 the greatest extension of the Swiss glaciers, the Lake of 

 Constance, and all the other great lakes, were filled with ice, 

 so that gravel and mud could pass freely from the upper 

 Alpine valley of the Rhine, to the lower region between Basle 

 and the sea, the great lake intercepting no part of the 

 moraines, whether fine or coarse. On the other hand, the Aar, 

 with its great tributaries the Limmat and the Eeuss, does not 

 join the Rhine till after it issues from the Lake of Constance ; 

 and by their channels a large part of the Alpine gravel and 

 mud could always have passed without obstruction into the 

 lower country, even after the ice of the great lake had melted. 



It will give the reader some idea of the manner in which 

 the Rhenish loess occurs, if he is told that some of the earlier 

 scientific observers imagined it to have been formed in a vast 

 lake which occupied the valley of the Rhine from Basle to 

 Mayence, sending up arms or branches into what are now the 

 valleys of the Main, Necker, and other large rivers. They 

 placed the barrier of this imaginary lake in the narrow and 

 picturesque gorge of the Rhine between Bingen and Bonn : 

 and when it was objected that the lateral valley of the Lahn, 

 communicating with that gorge, had also been filled with loess, 

 they were compelled to transfer the great dam farther down, and 

 to place it below Bonn. Strictly speaking, it must be placed 

 much farther north, or in the 51st parallel of latitude, where 

 the limits of the loess have been traced out by MM. Omalius 

 D'Halloy, Dumont, and others, running east and west by 

 Cologne, Juliers, Louvain, Oudenarde, and Courtray, in 

 Belgium, to Cassel, near Dunkirk, in France. This boundary 

 line may not indicate the original seaward extent of the 

 formation, as it may have stretched still farther north, and its 

 present abrupt termination may only show how far it was 



