NEW ENGLAND APPALACHIAN SYSTEMS 



161 



Fig. 11.8. Cross section along Kenisco bypass tunnel of the Delaware aqueduct. Kenisco Dam is 

 just east of Croton Lake in the lower Hudson Valley. Reproduced from Geological Sociefy of America 

 Guidebook of Excursions, 1948. 



of two samples is given as 400 and 440 m.y. The authors suggest that the 

 Fordham gneiss being demonstrably older and probably the Precambrian 

 basement did not lose all its argon during the 365 m.y. recrystallization 

 I process, and hence its micas yield somewhat older dates. It will be re- 

 called that zircons from the Baltimore gneiss of the crystalline Piedmont 

 yielded ages of about 1100 m.y., whereas the micas from the same rock 

 gave ages of 300 to 350 m.y. 



The age of the sediments themselves is not indicated by the isotope 

 age determinations but, at least, the time of the last major orogeny and 

 metamorphism is sufficiently young so that the sediments could well be 

 Cambro-Ordovician. 



Green Mountains 



The Hudson and Housatonic highlands, if followed northerly, lead 

 to the Taconic Mountains and northeasterly to the "western highland" of 

 Connecticut and Massachusetts, of which the Berkshires are a part. See 



Precambrian area in western Massachusetts, Figs. 11.2 and 11.9. East 

 of die western highland is the Triassic lowland. The Berkshire Mountains 

 extend to the Green Mountains at about the Massachusetts and Vermont 

 border, and the Green Mountains continue northward through central 

 Vermont to Quebec. See Cady, 1960. The Taconic Range extends 

 northerly along the New York-Vermont border to about central western 

 Vermont, and between it and the Berksire-Green Mountain element is 

 the "marble belt." For the broad relations of these geologic units see the 

 tectonic map, Fig. 11.10. The Green Mountains are comparable in eleva- 

 tion with the Adirondacks which lie across the Lake Champlain Valley, 

 but the other highlands and ranges are comparatively low. 



The core of the southern end of the Green Mountains is made up of 

 granites and gneisses of Precambrian age. These ancient rocks are over- 

 lapped on the flanks by quartizites of lowest Cambrian age. See lower 

 cross section of Fig. 11.11. The northern part of the range is a gneiss and 

 schist anticlinorium which plunges northerly, and although somewhat like 



