Shaler.] 358 



On the Formation of the Excavated Lake Basins of 

 New England. By N. S. Shaler. 



No phenomena resulting from tlie action of the forces in operation 

 during the Drift period are so enigmatical as the excavated lake 

 basins found throughout nearly the whole of the regions to the north 

 and south of the equator which present distinct evidences of glacial 

 erosion. Wherever found, the similarity of form seems to evince the 

 essential similarity of the forces involved in their production, and the 

 striking contrast they afford to all the ordinary results of erosive ac- 

 tion compels us to seek their origin in some cause or causes which 

 affected only regions on which the glacial sheet was imposed. 



There are in operation in the regions characterized by glacial 

 lakes, no forces capable of producing such depressions ; on the con- 

 trary all the forces at present in action tend to obliterate the existing 

 basins. This fact needs to be borne in mind if we would comprehend 

 the full extent of the facts, for over the surface of New England, and 

 probably over all such lake countries, sedimentary accumulations and 

 the formation of peat bogs have diminished the original area of the 

 basins quite one half A very large portion of them have lost the 

 character of lakes, and thereby ceased to be conspicuous features in 

 the landscape, so that it is only after careful examination of the struc- 

 ture of a region that the original extent and number of these peculiar 

 basins can be clearly perceived. If we could expose the surface of 

 the rock on which the glacial mass rested, throughout New England, 

 we would i^robably find no considerable area which did not present 

 basins referable to glacial erosion alone. Over the surface of Eastern 

 Massachusetts, where the observations of the author have mostly been 

 made, it is not easy to find a space of ten square miles which does not 

 present unmistakable evidence of this local erosion. The occurrence 

 of these basins over such wide spread areas, and their existence on 

 surfaces at considerable distances from steep declivities, renders it 

 highly improbable that they could have been produced by the local 

 erosion which takes place where a glacier meets a comparatively plain 

 surface after passing over a steep slope. This action, though compe- 

 tent to produce basin-like depressions, is manifestly insufficient to 

 account for the majority of the cases. The theory which assigns irreg- 

 ular upheaval as the cause of these basins, is not applicable, since the 

 contour of the basins and the structure of the rocks about them prove 

 conclusively that in most cases they are due to excavating agents. 

 Nor has it been shown why such abnormal elevatory actions are 

 restricted within the subglacial area. 



There are some facts connected with the distribution and form of 

 glacial basins which have an important bearing on all theories of their 



