s 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



features in the region constitute a mountain system. In working over 

 the structural features of North America I find that the divisions of the 

 major structural provinces can fittingly be called ranges and that many of 

 the major provinces themselves, systems. Range, therefore, will be used 

 to denote a sharp uplift about 10 to 75 miles wide and 50 to 200 miles 

 long. Commonly the structure is an asymmetrical anticline. In some, the 

 steep flank has broken into a high-angle fault or a thrust. Others may 

 consist of several folds or even thrust slices. Probably all ranges that were 

 eventually buried suffered considerable erosion beforehand. 



Platform and Shelf 



The terms platform and shelf in a structural sense are logically used 

 by King ( 1942a ) in the Permian area of west Texas and southeastern New 

 Mexico. There previously existing range-sized uplifts were buried, and 

 as sediments continued to accumulate, the adjacent basins were depressed 

 more than the old uplifts, so that although sediments accumulated on 

 the uplifts themselves, broad anticlinal structures developed over them. 

 These are called platforms. Beyond the basins, shallow seas existed, but 

 the crust subsided much more slowly there than in the basins, and a 

 much thinner deposit of sediment accumulated. These are called shelves. 

 A platform is similar to a shelf in regard to thickness of sediments on it, 

 but much more restricted in size and bounded on the two sides by ba- 

 sins. This is the sense in which the terms will be used in the following 

 pages. 



Welt and Furrow 



Bucher (1933) defines welt and furrow as crustal elevations and de- 

 pressions that show a distinct linear development. No special size or 

 origin is implied. A welt may be as large as a great deformed geosyncline; 

 viz., note Bucher's reference to Hobbs's phrase, "the gigantic welt of the 

 Himalayas." In Bucher's analysis of the deformation of the crust on a 

 world-wide scale, he needed these noncommittal terms, but in the present 

 attempt to picture the structural evolution of the North American conti- 

 nent, the names do not seem necessary, and they will not be used. 



Hinterland and Foreland 



Hinterland and foreland are terms introduced by the European ge- 

 ologists to distinguish the landmass or resistant elements of the earth's 

 crust on either side of an orogenic belt. In the Alps great, intricately 

 folded masses of sediments of the geosyncline, plus injected rock, moved 

 northward many miles. The north stable land toward which they were 

 moved is called the foreland, and the landmass south of the geosyncline 

 is called the hinterland. In the main, the great thrust sheets of the Ap- 

 palachian and Rocky Mountain orogenic belt have overridden toward the 

 interior stable part of the continent, and this ( at least the parts adjacent 

 to the orogenic belts) has generally been called the foreland. The land- 

 masses or borderlands on the oceanward side have been referred to as the 

 hinterlands. It is apparent that confusion must arise in the use of the 

 terms when some thrust sheets have overridden toward the oceans and 

 when, perhaps, no great, stable borderland existed. Some geologists also 

 contended that outward from the continent is the foreland. As for usage 

 in this book, foreland will mean the part of the stable interior adjacent to 

 a marginal orogenic belt, and lands oceanward of a marginal trough of 

 sedimentation, created by previous orogeny and from which sediments 

 were derived will be called the hinterland. 



TERMS FOR STRUCTURAL DISTURBANCES 



Revolution and Synonyms 



The term revolution is deeply intrenched in geologic literature, al- 

 though a number of authors, both here and abroad, have avoided its use, 

 and one has recommended its abandonment (Spieker, 1946). 



Schuchert's (1924) definition of a revolution is more complete than any 

 found, and characterizes many usages of the term. 



Near the close of the eras . . . occur the most extensive times of mountain 

 making, . . . These times of major diastrophism are the critical periods or 

 revolutions in the history of the earth, and they divide, as it were, the book 

 of geologic time into chapters. The critical periods are marked by the fol- 

 lowing features: 



