172 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



is the principal structural feature of the bedrock of Vermont, extends north- 

 northeast from the Massachusetts- Vermont border the full length of the state 

 and about 50 miles into Quebec, a total distance of about 210 miles. The 

 stratigraphic sequence and lithologic character of the rocks on the west limb 

 of the anticlinorium are different from those on the east limb, and a generally 

 accepted correlation of the two is not yet possible. In the Hyde Park quad- 

 rangle, and in the Montpelier quadrangle (Cady, 1956), which borders on 

 the south edge of the Hyde Park quadrangle, the general eastward dip of 

 the rocks is interrupted by a group of anticlines whose axes parallel the axis 

 of the Green Mountain anticlinorium. [See Fig. 11.18.] 



The bedrock of the quadrangle (s) comprises chiefly metamorphosed sedi- 

 mentary and volcanic rocks, principally schist, phyllite, slate, granulite, 

 quartzite, greenstone, amphibolite, crystalline limestone, and conglomerate, 

 that range in age from Cambrian probably to Devonian. Intrusive igneous 

 rocks, some of which are metamorphosed, underlie less than 1 percent of 

 the area and comprise serpentinite and its derivatives (talc-carbonate rock 

 and steatite ) , granite, and diabase that range in age from Ordovician probably 

 to Mississippian. 



All the rocks in this erea except the lamprophyre dikes have been affected 

 by regional metamorphism. In this area, chlorite, garnet, and kyanite have 

 been interpreted as successively general indicators of increasing metamorphic 

 grade in the schists. Similarly, chlorite, actinolite, and hornblende are indicators 

 in the greenstone and amphibolite. Most of the Hyde Park quadrangle is in 

 the chlorite zone of metamorphism (Cady, 1956). 



Bodies of serpentinite or its alteration products, talc carbonate rock and 

 steatite, are numerous, having been noted in fifteen places by Albee and 

 in five by Cady. They occur chiefly in the Stowe formation. 



The serpentinite (or its derivatives) forms tabular, lenticular, or pod- 

 shaped masses that strike north-northeast and dip steeply, parallel with the 

 schistosity and commonly also with the bedding of the enclosing rocks. The 

 serpentinite is dark green to dark greenish black on the fresh surface but 

 weathers to a characteristic pale greenish-white or light-buff rind traversed 

 by a reticulate system of sharply cut lines; it is composed almost entirely of 

 the mineral serpentine, probably of the antigorite variety. The talc-carbonate 

 rock is mottled greenish gray and weathers brown; it is composed of the 

 minerals talc, magnesite, and locally small amounts of dolomite. The steatite 

 ranges from white to green and greenish gray and weathers grayish tan; it 

 is composed of the mineral talc (Albee, 1957). 



Thick sills of granite invade the Waits River formation of the Mont- 

 pelier quadrangle, and have generated cordierite and diopside as contact 

 metamorphic effects. These sills are probably a late element of the 



Acadian folding which took place in Mid- and Late Devonian time 

 (Cady, 1956). 

 The minor folds do not accord with the major folds. 



The axes of most of the minor folds and granular quartz columns, as well 

 as the intersections of fold bands and of slip-cleavage lamellae with bedding, 

 are nearly vertical. This attitude implies that most of these minor structural 

 features were not produced by shearing movements in a nearly east-west 

 oriented vertical plane, such as were evidently responsible for the gently 

 plunging structures of the Green Mountain anticlinorium. Instead they were 

 probably either formed before folding of the anticlinorium by shearing move- 

 ments in a north-south vertical plane, or after folding and tilting of the limbs 

 of the anticlinorium by shearing movements in a north-south vertical plane, 

 or after folding and tilting of the limbs of the anticlinorium to near vertical, 

 by shearing movements in a horizontal plane. The pattern of movement of 

 these minor folds is uniform over rather wide areas; thus most of the folds in 

 the fold bands in the Moretown formation southeast of the Worcester Mountains 

 are dextral in plan (see White and Jahns, 1950, p. 197, for usage of terms 

 "dextral" and "sinistral"), and it appears that the rocks to the east have moved i 

 south relative to those to the west. This relationship is well shown at the 

 previously cited exposures of the Moretown formation in Middlesex Gorge 

 (Albee, 1957). 



Central and East-Central Vermont 



The outcrop pattern of three key formations in central and eastern 

 Vermont is broadly shown on the map of Fig. 11.20, and the stratigraphic 

 succession in Fig. 11.18. According to White and Jahns: 



The formations of central and east-central Vermont are exposed as a series 

 of parallel belts that strike nearly north. Most of the rocks dip steeply, and 

 many are overturned. With one possible exception, there seem to be no 

 major repetitions within the sequence, and the order of formations from 

 west to east appears to be the same as the order of their deposition. The 

 formations are dominantly schist or phyllite, with varying proportions of 

 arenaceous material. One thin formation, the Shaw Mountain, contains quartz 

 conglomerate, calcareous tuff, and crinoidal limestone. The third-from-highest 

 formation, the Waits River, is very thick and contains a large proportion of 

 calcareous beds. The distance from the base of the lowest formation to 

 the top of the highest, measured normal to bedding, is more than 100,000 

 feet; this large apparent thickness is believed to be not very much greater 

 than the original thickness. 



The metasediments have been intruded by granitic dikes and plutons 



afic dikes, and small ultramafic plutons. 



^Yn 



