1856.] Physical Geography of North America. 177 



palaeozoic strata resting upon them. Its entire surface is covered 

 with northern pleistocene drift, to so great a depth in some tracts 

 as effectually to conceal the formations underneath. The limpidness 

 of its streiuns and lakes must be attributed mainly to the filtering 

 action of this sandy and gravelly boulder drift, which contains but 

 little dispersed clay, though beds of pure brick clay regularly 

 stratified do occasionally occur in it. 



The south-western section of the Atlantic slope, expanding 

 south-westward from the Hudson, attains in Virginia a width of at 

 least 200 miles, under which it continues to the southern end of 

 the Appalachians. It includes two parallel belts, quite distinct in 

 their geology and scenery : that nearest the mountains is a true 

 slope, descending from a height of several hundred feet to the level 

 of the tide ; that bordering the sea is a low monotonous and very 

 level plain. Between the slope and the plain runs a well defined 

 line of demarcation, indicated in a sudden change in the topography, 

 and in the abrupt transition in all the streams, from rapids to the 

 ebb and flow of the tide. Upon this physical boundary are seated 

 all the chief cities of the Atlantic sea-board, from Trenton, in New 

 Jersey, to Halifax, in North Carolina. This line marks, indeed, an 

 ancient sea coast of the earlier continent in the times anterior to 

 the elevation of the horizontal tertiary deposits. 



The upper, or Appalachian division of the Atlantic slope, traced 

 south-westward, ascends from the level of the tide at the Hudson, 

 to its maximum height of 1000 feet near the sources of the Roanoke, 

 and declines again beyond the Catawba, to a more moderate level in 

 middle Alabama. From Long Island Sound to the Potomac its 

 surface is varied and pleasing, and its soil very fertile. 



Geologically, this belt includes tracts of the gneissic series, and 

 synclinal troughs of the most ancient palaeozoic formations — all in a 

 more or less metamorphic condition. These are overlaid by a zone 

 of middle secondary red sandstone, of older Jurassic age. This zone 

 embraces low rugged wooded ridges, and hills of trappean and other 

 gneous rocks. It possesses many broad, fruitful, and salubrious 

 valleys ; is singularly well watered by clear running brooks, but is 

 nowhere marshy, and is altogether the garden spot of the Atlantic 

 border of the continent. 



From the Potomac, south-westward, the features of this slope 

 are more monotonous, the soil less fertile, consisting of light sands 

 and meagre clays, produced from the subjacent talcose and chloritic 

 schists, and other altered azoic rocks. The climate is less temperate, 

 being liable to great heat in summer, and to heavy rains, which wash 

 its pulverulent soil. These conditions of soil and climate impart 

 excessive turbidness to its swiftly flowing streams, causing indirectly 

 extensive sand bars and shoals at the mouths of its rivers and estua- 

 aries, tending to block the navigation. Immediately at the base of 

 the blue ridge, in Virginia and North Carolina, this region contains 

 highly fertile and picturesque tract. 



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