DEFORMATION OF SHOEE LINES. 517 



of the planetesimal hypothesis of Ckaniberlin and that of continental crustal creep with 

 peripheral folding as formulated by Suess. 



CRUSTAL CREEP. 



The greatest earth movements involved in the formation of continents and mountains 

 are mainty horizontal or tangential creeping and thrusting movements, the associated move- 

 ments of elevation and subsidence being mainly incidental or secondary. Hence deformations 

 due to tangential or nearly horizontal creeping movements, such as Suess found to be involved 

 in the formation of the mountain ranges of Asia and in the growth of the continent itself, are 

 fully competent to be the cause of elevation and warping of old shore lines. 



The writer has endeavored to apply to North America the principles evolved by Suess 

 in Asia 1 and has found two widely separated sets of phenomena that clearly indicate a crustal 

 movement of the continent toward the southwest. One of these is the mountain knot of 

 Alaska, the highest mountain mass in North America, located at the angle where the Aleutian 

 and Cordilleran mountain ranges come together on converging lines. The mountain ranges and 

 the knot are mainly of Tertiary age. The other feature is the rift valley on the northwest and 

 west sides of Greenland, probably mainly of Tertiary age also. The bearing of these two 

 features on the direction of crustal movement of the continent is clear and single in its meaning 

 and is not susceptible to two opposite interpretations. In this respect they stand in strong 

 contrast with the usual explanations of overthrusts and folds, and even a group of folds showing 

 diminishing intensity from one side to the other, which are nearly always susceptible to two 

 opposite interpretations as to the direction of the main deep-seated folding force. This arises 

 from the difficulty of distinguishing relative from actual movements. In the Rocky Mountain 

 region what have generally been called northeastward overthrusts may just as truly be 

 described as southwestward underthrusts with northeastward overthrusts as secondary results. 

 Suess found the most intense folding at the front of the southward creeping crustal sheet in 

 Asia, where this sheet met a resistant obstacle — either a foreign mass, like India, or more often 

 the upturned, metamorphosed, basal layers of the crustal sheet itself. On the other hand, 

 the mountain knot of Alaska and the Greenland rifts are neither of them open to two opposite 

 interpretations; they are facts of two different classes and relate to distant and opposite sides 

 of the continental sheet, and yet they indicate the same direction of crustal creep. It is per- 

 haps worth while to point out that the recently uplifted and tilted region of the Great Lakes 

 and the extension of this deformed region to the northwest across the continent follows a 

 course substantially parallel with the great disjunctive line or rift on the west side of Greenland 

 and is also in part subparallel with the Tertiary mountain belt of the Pacific coast. All of 

 the region east of this belt has been affected by the same recent uplifting movement, and its 

 southwestern border, passing through the Great Lakes region and far to the northwest, appears 

 to stand either in the relation of a broad, low swell or anticline or else a broad, low monocline 

 with gentle dip to the southwest. It occupies a medial position between the Greenland rift 

 and the Cordilleran range, and its axis is transverse to the direction of the southwestward 

 crustal creep which these two features indicate. Its position is therefore natural for a fold 

 produced by crustal movement in the direction indicated, and such movement seems to the 

 writer to be the only one that would explain the uplift as part of a definite, systematic process 

 of continental development. 



The recurving of the marine isobases south and east of Quebec (fig. 14) suggests that a 

 broad, low anticlinal fold is there pitching downward to the east. This may or may not be 

 true, for, as was pointed out above, the raised beaches near Quebec probably do not record 

 all of the recent uplift in that region, and the complete record might show a different config- 

 uration of the isobases. But from the eastern end of Lake Ontario the isobases run west- 

 northwest at least to some point beyond Manitoba, and if an anticlinal fold exists its axis lies, 

 in all probability, north of the Great Lakes and parallel with the isobases now determined. 

 A broad belt west and northwest of Hudson Bay, including the region of the Archean rocks and 



1 Bearing of the Tertiary mountain belt upon the origin of the earth's plan: Bull. Geol. Soc. America, vol. 21, No. 2, 1910, pp. 179-226. 



