NEW ENGLAND APPALACHIAN SYSTEMS 



169 



structure of the area between Snake Mountain on its west limb and the 

 Green Mountain front on its east limb. The center of the synclinorium is 

 covered by the great Taconic klippe south of the latitude of Brandon. 

 The east limb may be traced fairly continuously into the marble belt south 

 of this latitude (Cady, 1945). The west limb loses its identity in an area 

 of high angle faults southwest of Orwell. The nature of the numerous 

 small folds of the synclorium are best shown in the cross sections B and 

 C of Fig. 11.11. 



The northern synclinorium, known as the Hinesburg, composes the 

 structure of most of the area between Lake Champlain and the Green 

 Mountain front. See section A, Fig. 11.11. Most of the east limb is covered 

 by the Hinesburg-Oak Hill thrust slices. The Hinesburg synclinorium is 

 not so symmetrical as the Middleburg synclinorium, and the folding is 

 limited to the development of a series of moderately broad basin struc- 

 tures (Cady, 1945). 



The normal faults of the Adirondacks have already been described. 

 The eastern border of the crystalline mass is formed in part by these 

 faults, and they seem to be genetically related to the uplift of the dome. 

 They do not intersect the major thrusts of the Lake Champlain region, 

 but they parallel the Orwell and Champlain thrusts, and for a distance 

 the bends in the normal faults coincide with bends in the thrust fronts. It 

 lis suggested (Cady, 1945) that the thrust fronts may have retreated by 



erosion eastward after they were trimmed by the normal faults, and thus 

 the parallelism has resulted. 



Tectonic History 



Champlain and Magog Troughs. In 1923 on the occasion of his presi- 

 dential address on North American geosynclines, Schuchert postulated a 

 western trough, the St. Lawrence, through the Lake Champlain and St. 

 Lawrence region, a medial divide or geanticline, and then an eastern 

 trough, the Acadian, principally through Nova Scotia and New Bruns- 

 wick. The geanticline included the Green Mountains of Vermont and 

 the White Mountains of New Hampshire and Maine. The rocks of these 

 mountains were then regarded as Precambrian. Since then several groups 

 of fossils have been found, and most of the metamorphosed sediments 

 of Schuchert's geanticline have turned out to be Lower and Middle 

 Paleozoic in age. Still two troughs seem necessary, but the eastern one 

 must have occupied approximately the site of Schuchert's geanticline. It 

 has been called the Magog eugeosyncline by Kay ( 1942), and the western 

 has been called the Champlain miogeosyncline. The Magog is character- 

 ized by shales, cherts, and various volcanics, the western by carbonates. 

 Until Mid-Ordovician time, the separation of the two troughs was prob- 

 ably a matter of facies, but then a land barrier called Vermontia rose 

 within the western part of the Magog trough and caused the deposition of 

 elastics over the carbonates of the western trough. See Fig. 11.17. Later in 



: 



WEST ERA! 

 NX 



SITE OF UPPER DEVONIAN "DELTA" 



MIOGEOSYNCLINE 

 Lower Devonion'} S-) 



, VT i \SOUTH 

 NYMUASS. VT.W.H N.H\MAINE GULF OF MAINE 

 -+- ' --LATER ACADIAN BELT- *• 



ATLANTIC 



VERMONTIA 

 GEANTICLINE . EUGEOSYNCLINE 



Lower Devonion'; 



APPALACHIA 





Fig. 11.17. Basins of deposition across New England just prior to Acadian orogeny. Compiled 

 from Kay (1951), Billings (1956), and other sources. Vermontia had risen in Mid-Ordovician 

 time and evidently was considerably wider than present dimensions indicate to supply 

 the voluminous elastics to the miogeosyncline in Mid- and Late Ordovician time. Vermontia as 



---0 _---' 

 Grenville orogenic complex 



shown was also essentially the site of the Taconic orogeny at the close of Ordovician time. The 

 eugeosyncline was the site of much volcanism, and Vermontia the site of ultramafic intrusions, cm, 

 Cincinnatian; moh, Mohawkian; and ch-can, Chazyan and Canadian. The region of Vermontia in 

 places probably received Silurian and Devonian sediments, so its history and nature is complex. 



