390 HISTORICAL GEOLOGY. 



bowlders, and terminal and lateral moraines gives a fas- 

 cinating interest to travel among these mountains. 



Contrast of Northern and Southern Soils and 

 Bock-Surfaces. Nothing can be more striking than the 

 contrast between the soil and underlying rock-surfaces 

 within and beyond the limits of the Drift. Within these 

 limits the covering is a heterogeneous mass of shifted ma- 

 terial lying on sound rock ; south of this limit the soil is 

 stratified, and in many places graduates into the rock be- 

 neath from which it has been formed by rotting in place. 

 Again, the underlying rock in drift-regions is glaciated, 

 i. e., smooth, moutonneed, scored; beyond the drift-region 

 there is either no distinct surface to the rock, or else, if 

 there be, it is a rough, weathered surface. 



2. Cliamplain Epoch. 



At the end of the Glacial epoch, when the condition 

 of things was such as described above, there commenced 

 a crust-movement in a contrary direction, by which the 

 land in the same region was brought downward 100 to 

 500 or 1,000 feet below this present level, and the lower 

 parts of the continent became covered with the sea. It 

 was therefore a period of inland seas. The movement was 

 attended with moderation of temperature, by which the 

 ice-sheet was melted and progressively retired northward. 

 The melting ice produced flooded lakes and flooded rivers. 

 It was therefore also & flooded period. Icebergs, loosened 

 from the northern ice-foot, floated over the inland seas 

 and the great flooded lakes, dropping debris. Some of 

 the great bowlders are probably to be accounted for in 

 this way. It was therefore also a period of iceberg agency. 

 The evidences of this condition of things are found in old 

 elevated sea-margins, lake-margins, and old river flood- 

 plain deposits. 



Sea-Margins. Elevated sea-beaches are found in all 

 countries affected with the Drift. The highest one marks 



