GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 



55 



the gneisses. Along the western margin of the strip the compres- 

 sion brought the Upper Cambric beds against the slates, which were 

 first folded and overturned and then overridden. At the quarry 

 at Stoneco and below Marlboro station across the river the west- 

 ward dipping younger strata show a diminution in the upward thrust 

 toward the southwest which may be associated with an earlier re- 

 lease that elevated the Glenham belt. 



As already indicated, the conglomerate appears in places along 

 the western margin of the strip. It is best interpreted as belonging 

 with the downthrow block and is to be associated with the slate 

 rather than with the limestone. It is likely that there was a strong 

 horizontal component in the thrust that carried the older beds over 

 the slates to the west of this strip. 



The conglomerate along the eastern margin would appear to 

 occupy a normal position, but the fact that it is brecciated is note- 

 worthy. The presence of ledges yielding Lingulepis pinni- 

 f o r m i s, as described above (see page 49), along the eastern 

 margin in the near neighborhood of the Trenton, seems best ex- 

 plained as an instance of faulting. The Potsdam beds seem clearly 

 to have been overlain by younger strata, as is now the case at 

 Stoneco quarry. It does not seem possible from the relationships 

 exhibited elsewhere that the overlying strata were eroded so as to 

 expose the Potsdam before the deposition of the Trenton. 



Apparently the 



limestone on the west 

 of the Hudson is es- 

 sentially the continu- 

 ation of this western 

 strip, but presumably 

 the beds are younger even than those of the quarry at Stoneco.. 

 Some of them resemble the beds of the central strip, as shown at 



the New Hamburg 

 tunnel. On the west 

 of the Hudson, near 

 the river's edge at 

 Danskammer, the 



limestone dips to the 

 southeast at an angle 

 N.vy. of 10°. Along the 

 western margin, as 

 shown just below Marlboro station, the dip is to the northwest. 

 The limestones rest by overthrust on the slates at the west. 



N.W. 



Fig. 



Section at Danskammer 



Fig. 21 Section below Marlboro station 



