362 PHYSICAL GEOLOGY 



A brief and incomplete history of a folded region is shown in Figure 

 351. It is incomplete because many of the important chapters of 

 the history cannot be shown without too great confusion of detail. 



3. Rate of Folding. — The rate of folding must necessarily differ 

 widely in different geosynclines and in the same geosyncline at vari- 

 ous times. In certain cases it seems to be proved that rivers have been 

 able to deepen their valleys as rapidly as the. land surface was elevated 

 (antecedent rivers, p. 102). It is possible that the general denudation 

 of a region may in some cases have proceeded at about the same 

 rate as the elevation, so that at no time was the surface far above 

 sea level. This may, for example, have been true of the Appalach- 

 ians, which now consist of comparatively low mountain ridges 



Fig. 352. — Section across central New England, showing the uplifted peneplain 

 and Mt. Monadnock. (Hitchcock.) 



although three or more miles of sediment have been removed by 

 erosion ; and also of the folded and crumpled rocks of New Eng- 

 land (Fig. 352). 



The elevation of regions of folding was not always continuous, 

 but, as is shown by a study of the Appalachians, the folded belts 

 were at times above sea level and suffered erosion ; upon being again 

 depressed they received more sediment, unconformities marking 

 the sites of the ancient erosion surfaces ; and later, they were further 

 folded and faulted and raised above the sea. The present height 

 of the Appalachians and Sierra Nevadas was brought about by 

 vertical elevation and not by lateral compression. 



4. To What the Topographic Features of Folded Mountains are Due. 

 — A comparison of the external form of mountains and their geo- 

 logical structure shows that the two seldom agree. It is true that 

 the mountain ranges in general are parallel to the strike (p. 353) of 

 the strata, but the valleys seldom coincide with the synclines and 

 the ridges with the anticlines. This coincidence sometimes occurs, 

 but the reverse is as frequently seen. In the Jura Mountains, 

 Switzerland, excellent examples of synclinal valleys may be seen 



