336 B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTANA 



upper part of the Siyeh limestone, and the Altyn limestone at the Lewis 

 thrust the thickness of strata is something more than 8,000 feet. 



STRUCTURAL ANTECEDENTS OF LEWIS THRUST 



Explanation. — By structural antecedent the writer means those earlier 

 relations of rock masses from which an existing structure has developed. 

 Thus an overturned anticline is one usual antecedent of an Appalachian 

 thrust fault. The Lewis overthrust is a result of conditions which can 

 now be stated hypothetically only, but which so stated may aid future 

 investigation to a truer understanding. To this end the following 

 hypothesis of its antecedent phases is presented : 



Assumptions. — Certain general assumptions may first be stated. The 

 surface of the overthrust is essentially parallel to the bedding of the 

 Algonkian series, and in this particular district to the Altyn formation, 

 where the latter has not been dislocated by minor thrusts. This appar- 

 ently is true not only of the segments of thrust surface beneath eastern 

 Flattop, Yellow, and Chief mountains, but also of the more deeply 

 buried portion which appears to dip down with the Algonkian strata 

 into the S}mcline. While observation is not complete, it may be assumed 

 on a basis of fact that thrust surface and bedding are nearly parallel over 

 extensive areas. 



As regards structure of the Cretaceous rocks, it is not found that the 

 thrust surface coincides with their bedding or any other internal feature 

 of their mass. But, with reference to physiographic features, it was ob- 

 served that the thrust plane was apparently continuous with the highest 

 peneplain of the Plains — that is, with the Blackfoot plain, the peneplain 

 which is cut on the upturned edges of the Cretaceous strata. As illus- 

 trating this relation, figure 1, plate 47, may be .described. On the right 

 is East Flattop mountain, as it appears when one is looking south 

 across Swift Current valley. It is composed of Algonkian strata, in 

 which a white quartzite shows the nearly horizontal attitude. At the 

 base of the cliff, just above the tree-covered slope, is the position of the 

 Lewis overthrust. The wooded slope consists of Benton shales, exten- 

 sively covered b}> r drift. On the left is Saint Mary ridge, the even crest 

 of which is somewhat built up as a lateral moraine of Saint Mary glacier, 

 but which from this point of view corresponds closely with the profile 

 of the old peneplain. That plain is strongly represented in Milk River 

 ridge, 12 miles east of the brow of Flattop mountain. Its gentle rise 

 westward, about 100 feet to the mile, carries it into the thrust surface 

 beneath Flattop. 



This relation of the thrust surface to the peneplain is one of critical 

 importance as a means of determining the antecedents of the Lew T is 



