262 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



Weverton, and on the return it is theoretically crossed between the 

 Highlands of the Hudson river and New fork City, but its manifosta 

 Hon there is not characteristic. 



The Blue ridge overlooks the Piedmont district from the no.-thwest, 

 and is a nearly continuous upland from New England to Georgia. At 

 most points it is a single ridge rising 2,000 feet above adjacent low- 

 lands, but in North Carolina it is expanded into a mountain chain, with 

 greater altitude. The outward journey intersects it in approaching 

 Harpers Ferry; the return journey, at the Highlands of the Hudson. 



The Appalachian valley is a wonderfully persistent belt of low land 

 separating the Blue ridge from the Appalachian mountains. For 

 Several geologic periods the Appalachian region has stood at so low a 

 level that its streams have had small declivity, and mechanical erosion 

 has been slow. The chemical factor in erosion has thus acquired rela- 

 tive prominence, and the broad outcrop of lower Paleozoic limestone 

 which occupies the valley area has been degraded by solution until its 

 surface is far below the contiguous outcrops of sandstone and shale. It 

 is an undulating plain, sharply incised along principal lines of drainage, 

 and otherwise characterized by ••swallow holes" or ''limestone sinks." 

 and by eaves. The outward journey traverses it from Harpers Ferry to 

 North .Mountain Station; on the line of the return journey it is not 

 well distinguished. 



The Appalachian mountains* 1 consist of Paleozoic strata, from Cam- 

 brian to Carboniferous, which have been acutely folded and faulted. 

 The steeper limbs of the fold are usually on the northwest side. Anti- 

 clines are often pushed northwestward over synclines. and in numer- 

 ous instances this process has culminated in thrust faults. The prin- 

 cipal epoch of folding ended early in Mesozoie time, and during the lat- 

 ter part of the -Mesozoie the district, which then stood several thousand 

 feet lower than now, was degraded to the condition of a peneplain. 

 Subsequent uplift renewed the activity of the streams, and the district 

 was carved into a grand cameo, in which the topographic features 

 express the rock texture and rock structure in a peculiarly effective 

 manner. The outcrop of each series of soft rocks is recorded in a sys- 

 tem of valleys; each great bed of hard rock caps a ridge, but Qone of 

 the ridges rise above the level of the old peneplain, and all the greater 

 ridges have even tops expressing that factor of their history. On the 

 outward journey this belt is traversed from North Mountain to the 

 Monongahela river; on the return journey it is entered at Schenec- 

 tady, but its features are masked by Pleistocene deposits. 



The Appalachian folding diminishes in intensity northwestward until 

 finally the dips of the strata are gentle. Wherever a massive sand- 

 stone approaches the height of the old peneplain in the region of 

 gentler tolds ,t has been preserved in the form of a high table, usually 

 ■ Figures above the line refer to titles in Bibliographic lis, at end of Guide Book. 



