MOUNTAINS AND PLATEAUS 359 



ated at the junction of great earth segments or blocks where, as a 

 result of the crowding of the latter^upon each other as they are 

 drawn toward the center of the earth, the weak strata of geosynclines 

 are folded. 



1. Geosynclines. — The sedimentary strata of which folded moun- 

 tains are formed are very thick ; in the Appalachians, the thickness 

 is about 25,000 feet; in the Coast Ranges of California, 30,000 feet; 

 and in the Alps, 50,000 feet. When a stratum is traced to a distance 

 of even a few miles from the mountain chain, it is found that it rapidly 

 becomes thinner; the strata that have a thickness of about 25,000 

 feet in the Appalachians, for example,. are only about 2500 feet thick 

 in the Mississippi Valley. An examination of the rocks of moun- 

 tain masses often shows that many of them are of shallow water 

 origin, as the occurrence of conglomerates and sandstones testifies. 

 Ripple marks, sun cracks, and fossils afford similar evidence. The 

 presence of limestones, on the other hand, may indicate (p. 238) 

 that the water in which they were deposited was deep or far from 

 shore. The sediments that are being laid down in the seas to-day 

 are deposited in a belt extending from the shore line to a distance 

 usually considerably less than 50 miles (p. 237). Since there is no 

 reason to believe that the conditions of sedimentation in the past were 

 markedly different from those of the present, it is generally held that 

 the strata composing the great mountain ranges were laid down near 

 shore and, since many of them are of shallow water origin, that sink- 

 ing accompanied and for the most part kept pace with the deposi- 

 tion, the land rising as the geosyncline sank. Occasionally uncon- 

 formities occur, which indicate, as has been seen (p. 270), that eleva- 

 tion for a time interrupted the deposit of sediment. 



2. Lateral Pressure. — When sediments have accumulated in a 

 geosyncline to a depth of several thousand feet, those near the bot- 

 tom of the deposit are somewhat weakened by heat (p. 347), so that 

 they are compressed and thrown into folds when subjected to great 

 lateral pressure. The strata composing the Appalachian Mountains 

 of Pennsylvania, between Harrisburg and Tyrone (Fig. 348), were 

 compressed from a width of 81 miles to one of 66 miles; i.e., the 

 earth's superficial crust, upon being folded, was shortened 15 miles, 

 with a resulting mean elevation of three miles. It has been estimated 

 that, if the folds of the Alps were smoothed out, the strata would 

 cover an area 74 miles wider than the mountains do now, or about 

 twice their present width. The shortening of the Front Range in 



