88 PROCEEDINGS OF THE CHICAGO MEETING 



mile above the station of Clielan Falls. The elevation of bedrock at this point 

 and of striated bedrock in the' Columbia River Valley, near by, is approxi- 

 mately 700 feet. Nearly 400 feet of the water of Lake Chelan, then, is held in 

 by a drift-dam and the remainder lies ^ a rock basin. 



Presented by title in the absence of the author. 



ANCIENT ROCK DEFORMATIONS AND THEIR PRESENT EXPRESSIONS IN 

 WESTERN VERMONT "■ 



BY CLARENCE E. GORDON 



(Alxitract) 



In vi^estern Vermont and contiguous areas the geology is greatly complicated 

 by ancient deformations of the rocks due to compression. The formations in- 

 volved belong to: (1) Pre-Cambrian ; (2) Cambrian, chiefly Lower Cambrian; 

 and (3) Lower and Middle Ordovician. 



The stress lias found expression in various kinds of internal deformation 

 due to shearing strain, and in mass dislocation. It is not easy to apportion 

 these different effects among the several disturbances which the vocks of the 

 region have probably experienced, but the outstanding features due to com- 

 pression may with much probability be attributed to one great, dramatic series 

 of episodes in their history. 



Internal deformation shows in various ways, and mass dislocations occur as 

 folds and ruptures, which take the forms of small and large reverse faults and 

 thrusts. 



In the region of Lake Champlain, for many miles north and south, among 

 the islands and along the shores of the lake, various eroded and now more or 

 less detached masses of massive siliceous, magnesian, and calcareous rocks 

 which have been metamorphosed in varying degrees, but whicli as a rule have 

 not been violently folded, rest by thrust on other calcareous rocks and shales 

 and slates which are younger and which have been folded and tilted, while at 

 other places a quartzite, or quartzite-phyllite, formation rests by thrust appar- 

 ently on the same slate formation. 



There is evidence to show that some' of the various calcareous rocks just 

 mentioned as overlapping younger formations by thrust to the east of the lake 

 now rest on the quartzite-phyllite formation, which is probably of Lower 

 Cambrian age. 



A reasonable interpretation seems to be that a great thrust cut through the 

 Lower Cambrian basement of certain calcareous rocks belonging to different 

 horizons of the Lower Ordovician and into the limestones along an irregular 

 plane that emerged somewhere east of what is now the lake and drove the 

 mass above the plane westward to its present resting place on the Trenton- 

 "Utica" formation. As now eroded at some places, the contact of the Cam- 

 brian on the slates may be seen, while at others so-called "Potsdam," or Beek- 

 . mantown, or possibly Chazy or Trenton, rests on the slates. No estimate 

 seems possible of the eastward extent of the hidden overlap contact, but con- 

 siderable lateral displacement appears probable. 



^ Printed with the consent of the '\'ermont State Geologist. 



