168 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



GREEN 



M O U N T A I N 



Fig. 11.16. Tectonic map of west-central Vermont, after Cady, 

 1945. Ruled areas are Ordovician strata in the synclinoria. 

 Faults with knobbed bars are normal faults. 



same orogeny, have been mapped. All are interpreted as having moved 

 from east to west. The chief ones of these are the Champlain and Hines- 

 burg-Oak Hill. The Champlain thrust trends parallel to Lake Champlain 

 and extends from a point near the south end of the lake northward about 

 60 miles to the Canadian border. About 3 miles north of the border, near 

 the village of Rosenberg, it becomes obscure in a shale terrane (Cady, 

 1945). See Fig. 11.16. The thrust at the north end is known as the Rosen- 

 berg slice (sheet). According to Cady, near the south end: 



At Snake Mountain Lower Cambrian beds of the mountain proper are 

 thrust westward across and beyond Upper Cambrian and Beekmantown rocks 

 of the Orwell thrust plate onto the Middle Trenton limestones and shales next 

 west and structurally continuous with those found along the lake; the Champlain 

 thrust apparently truncates the Orwell thrust. 



The Hinesburg-Oak Hill thrust complex floors a tectonic unit east of 

 the Champlain thrust and it in turn is bounded on the east by the Green 

 Mountains. The southern part is called the Hinesburg thrust, and the 

 northern the Oak Hill. The Oak Hill thrust sheet passes beneath the 

 Hinesburg. 



The rocks of both the Hinesburg and Oak Hill thrust slices grade eastward 

 into the schist and gneiss terrane of the Green Mountains. Both of these slices, 

 so far as they have been delineated, apparendy have undergone considerable 

 displacement, as evidenced by the depth of erosional re-entrants and by the 

 outlying position of klippes. The Hinesburg and Oak Hill thrusts form the 

 eastern boundary of the Rosenberg slice. 



^ The rather highly deformed quartzose slates, phyllites, and graywackes east 

 of the Hinesburg thrust, a short distance north and east of Hinesburg village, 

 lie with angular discordance across the east limb of the Hinesburg synclinorium, 

 where the thrust plane truncates minor folds which are made up of beds from 

 Lower Cambrian to Beekmantown age. The thrust plane has not been observed 

 at any point, but the depth of the re-entrants suggests that it dips at a very 

 low angle to the east. Non-quartzose black slates and phyllites crop out west 

 of the quartzose rocks along the thrust front in St. George and Williston town- 

 ships. These latter Upper Cambrian argillaceous rocks comprise the Muddy 

 Brook thrust slice, which was apparently dragged up along the sole of the 

 Hinesburg thrust. These same slates and Upper Cambrian sandy dolomites 

 crop out in the re-entrant west of Williston village. Northwest of Williston 

 village the quartzose rocks are thrust over a closely folded syncline of the 

 Oak Hills slice. In this syncline are formations from Lower Cambrian to prob- 

 ably Upper Cambrian age. 



In general, the rocks east of the Oak Hill thrust are less deformed and less 

 uniform in appearance than those east of the Hinesburg thrust. The lower 

 Cambrian Dunham dolomite is everywhere recognizable, and at many places 

 along the thrust front, where structures involving the Dunham are truncated 

 at erosional re-entrants or at klippes such as Cobble Hill in Milton township, it 

 locates the fault. Where argillaceous rocks are near the contact, the fault is 

 much more difficult to locate, inasmuch as the eastern exposures of the 

 Rosenberg slice are in a predominandy argillaceous terrane (Cady, 1945). 



Two synclinoria lie on a common north-south axis and are separated 

 by the Monkton cross anticline. See Fig. 11.16. They are bounded on the 

 west by the Adirondack dome and Champlain thrust and on the east by 

 the Hinesburg-Oak Hill thrust and the Green Mountains. 



The southern synclinorium, known as the Middleburg, makes up the 



