46 THE GEOLOGIST'S TRAVELING HAISTD-BOOK. 



The usual characteristic marks of glaciers extend, according to Agassiz, over 

 the whole surface of the east half of the continent, from the Atlantic shores to the 

 states west of the Mississippi, and from the Arctic Sea to the latitude of the Ohio, 

 about the 40th degree of north latitude. The glacier marks tend from north to 

 south, with occasional slight inclinations to the east or west, according to the 

 minor irregularities of the surface. The ice of the great glacial period in America 

 is supposed to have moved over the continent as one continuous sheet, over-riding 

 nearly all the inequalities of the surface. The drift spread in one vast sheet over 

 the whole land, consisting of an indiscriminate medley of clay, sand, gravels, 

 pebbles, boulders of all dimensions, so uniformly mixed together that in all parts 

 of the country it presents hardly any difference. The total absence of stratification 

 is one important characteristic of glacial drift. There is no arrangement of the 

 materials according to size or weight, whereas in water the lighter materials are 

 carried farther than the heavier ones, and the heavier ones are at the bottom and 

 the lighter on top. In glacial drift there are large angular fragments by which it 

 may be distinguished from alluvium, and it retains the mud gathered during the 

 journey, and spread through its mass, while the water-worn deposits are washed 

 clean, and consist always of well-rounded pebbles, and there are no scratches on 

 the exposed surfaces of the solid rocks. 



There appeared in 1878 a map of New Jersey, on which the ice -covered area 

 of that state is laid down, with a description of the terminal moraine, extending 

 from below Belvidere, on the Delaware, first east and then south and southeast to 

 South Amboy, across Staten Island and through the middle length of Long 

 Island. Prof. Hitchcock has traced it thence eastwardly to Massachusetts Bay. 

 Ice covered the highest peaks of the northern mountains of New Jersey, as it did 

 all the mountain crests of Eastern Pennsylvania, to within ten or fifteen miles of 

 Harrisburgh. The southern limit of the moraine has been fixed, by Mr. Carll, at 

 Titusville, in Venango County, and by Prof. White, at Newcastle and Beaver 

 Falls, in Beaver County. There blocks of granite from Canada lie perched on 

 hill-tops of Coal Measures 1,300 feet above tide level. 



Much of the configuration of the country has been attributed to the action of 

 glaciers, but Professor J. P. Lesley has advanced the theory that most of the 

 topographical features of the Atlantic half of the United States, including the 

 erosion of Lakes Champlain and Ontario, and Georgian and Green Bays, the Blue- 

 grass country of Kentucky, the central basin of Tennessee, the great valley of 

 Eastern Tennessee, Virginia, Pennsylvania and New Jersey, the Taconic valleys 

 of Western New England, and the rich valleys of the interior parts of the 

 Appalachian Mountain belt, have been due, as he thinks, to the underground 

 dissolution of the Lower Silurian limestone formations, and to the consequent 

 breaking down of the Paleozoic roof above the caverns thus excavated ; the 

 process, however, beginning with the limestones of the carboniferous and sub- 

 carboniferous age, being continued by the second subterranean erosion of the 

 Upper and Lower Helderberg limestones, causing Lakes Erie, Huron and Michigan, 

 the smaller New York lakes,* and the "Poor Valleys" of the Middle and Southern 

 Atlantic States, and ending with the subterranean erosion of the Trenton and 

 Calciferous formations, which, he says, is, in fact, seen to be still going on. 



One of the most notable features of the Western States is the PRAIRIES, which 

 are vast natural meadows, sometimes hundreds of miles in extent, bare of trees 



*These and other features of Central New York, may have originated from the solution of large 

 beds of rock-salt in the underlying Salina formation, and the consequent subsidence of the strata. 



