IlOir DEPENDENT ON CHANGES OF LEVEL 169 



into the Dismal Swamj), where they receive httle or no 

 detrital matter even in times of heavy rain, occasionally retain 

 a depth of twenty or thirty feet, their bottoms lying even 

 beneath that of the neighboring parts of the open sea."" 



Few save those who have made a special study of geology 

 have any clear idea concerning either the nature or frequency 

 of those movements which take place in the shore districts of 

 the greater land masses termed continents. It therefore may 

 aid the reader to understand the extent to which havens are 

 formed by the submergence or flooding of old river valleys, if 

 he is presented witli some of the more important acts and con- 

 clusions concerning these curious movements. It is well at 

 the outset to understand, that although there is much evidence 

 to show that from the earlier geological ages the continents 

 have existed as areas of dry land, that of North America, for 

 instance, having been a geographic unit since the Carbonifer- 

 ous Period, or, at most, with its eastern and western parts 

 divided by a narrow sea, the outlines of this and other lands 

 have been subject to frequent and extensive changes of level. 

 Thus in the geologically brief period since the beginning of 

 the last Glacial Epoch, or, as we may fairly term it, since the 

 morning of the geologic yesterday, all the eastern coast of 

 North America has swayed up and down in several successive 

 oscillations ranging from a few score to a few hundred feet. 

 At the beginning of this last ice age the northeastern portion 

 of the continent was evidently somewhat higher than it is at 

 present. When the weight of the ice lay ui)on the land, it 

 sank down so that the depression along the coast of New 

 England as compared with the present level of the sea was 



* See General Account of the Fresh-water Morasses of the United States, by 



N. S. Shaler. Tenth Annual Report, pp. 327, et seq. 



