GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 35 



On the east of the Hook spur another fault of moderate dis- 

 placement has dropped the quartzite and limestone into the East 

 Hook. 



The northwestern slope of Shenandoah mountain is very steep 

 from the point where it cuts the southern boundary of the quad- 

 rangle nearly to Shenandoah. The quartzite has a northwest dip 

 of approximately 50°. The gneiss in places shows precipitous 

 ledges, though these are not very high. The angle of slope changes 

 abruptly from quartzite to gneiss. The steep dip of the quartzite 

 shows considerable disturbance before the break occurred. 



East of Shenandoah mountain is a clearly defined normal fault 

 scarp along which the younger rocks were dropped. Their erosion 

 has formed Shenandoah hollow. 



Along the northwestern slope of the eastern gneiss mass are very 

 steep and precipitous scarps, sharper even than those of Bald hill. 

 The drift-covered talus slopes at their bases are not to be confused 

 with the quartzite. It is probable, however, that in places the 

 quartzite was involved in the upthrow and was brought against the 

 limestone. 



These breaks are interpreted as the result, primarily, of the com- 

 pression producing the Green mountain elevation. The tendency 

 was to produce a system of flexures like those in the younger rocks 

 at the north. The gneisses buckled relatively little but, unable to 

 resist the great pressure, were broken and thrust up into the younger 

 rocks. Tension faulting within the expanded arc accompanied 

 or followed the upward thrusting. 



The faulting in the gneisses is clearly subsequent to the de- 

 position of the quartzite. The only disturbance capable of produc- 

 ing these effects would appear to have belonged to the close of 

 Ordovicic time. 



These faults would certainly have greatly disturbed any orderly 

 sequence which the gneisses may have had. 



SUMMARY AND CONCLUSIONS 



The relatively brief treatment of the gneisses of this quadrangle 

 given above results from the impossibility of assembling them 

 into an orderly sequence. The thickly-wooded character of the 

 country, the presence of faults and the difficulties introduced by 

 ancient subaerial alteration, greatly hinder their study and make a 

 satisfactory map practically impossible. 



