160 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



units are arranged in belts of north-southerly trend. The horizontal forces 

 that caused the thrusts are also believed to have cast the sedimentary 

 rocks into folds which are overturned to the west. The folds, however, 

 are very small ones in otherwise gently downfolded beds. 



Cleavage pervades the crenulated sediments widely. It is everywhere 

 parallel to the axial planes of the crenulations, and is best developed in 

 the slate phases northeast of Poughkeepsie. 



The metamorphic rocks of the lower Hudson River Valley have been 

 regarded as Precambrian, but in light of Ralk's and Barth's work it seems 

 probable that only the Fordham gneisses is Precambrian and that the 

 Inwood limestone is equivalent to the Wappinger limestone and the Man- 

 hattan schist to the Hudson River pelite, both of Cambro-Ordovician age. 

 Refer to cross sections of Figs. 11.5 and 11.8. For discussion of the prob- 



DENKIARK 



WEST CANADA 

 CREEK 



lem see Balk, 1937. Paige (1956) has correlated undoubted Cambro- 

 Ordovician rocks west of the Hudson River near Peekskill with the 

 Inwood marble and Manhattan schist east of the river. 



Potassium-argon age determinations on the micas of the Manhattan 

 schist, the Inwood marble, the Fordham gneiss, some discordant pegma- 

 tites, and a diorite were made by Long and Kulp (1958). An average age 

 for the generation of the micas of the post-Fordham gneiss formations is 

 given as 366 =*= 9 m.y., which they say may tentatively be correlated 

 with the Late Ordovician Taconic orogeny. Very recent interpretations 

 by Hurley et al. ( 1959 ) indicate that this absolute age may be post-Early 

 Devonian, and in connection with orogeny in New Hampshire their work 

 will be referred to again. 



Biotite from the Fordham gneiss is slightly older; the "apparent age" 



MOHAWK VALLEY 



WELL5 



LAKE CHAMPLAIN CANAJOHARIE 



z 



Zone 



or 



/} m /o/ ex og f* a jo f </sjtz-T~z. am r> / ' e. jt / c a <y / '/\s _r~ _ 



CAn AO, AN ' S ^^^~~^T 8=7 h 



[z 



^oJvv^ — £H O/7EHfl M * f H 



ADIRONDACK AYIS 0*p 



u 



a 

 o 



h 



vo 

 hi 



Fig. 11.7. Restored section of pro-Middle Trenton formations across 

 York, after Kay, 1942. 



