GEOLOGY OF THE NORTH CREEK QUADRANGLE 51 



Moon-Hackensack mountain fault. This is a well-defined fault 

 whose presence is proved by most of the criteria generally appli- 

 cable in this region. The strike is northwest-southeast and in per- 

 fect harmony with that of Moon and Hackensack mountains. The 

 trend of the fault scarp is clearly marked by the topography, the 

 western face of Moon mountain being especially high and steep 

 (see plate 9). Betw^een the two mountains the scarp is lowest 

 because the weaker Grenville there has been worn down most 

 rapidly. This whole earth block shows the eastward downtilt, 

 particularly in the case of ]\Ioon mountain. In a large prospect 

 hole at the western base of Hackensack mountain and close to the 

 road there is much evidence of shearing along the fault. Near 

 the source of the small stream along the fault and half way between 

 the mountains there is a fine exhibition of crushed and brecciated 

 Grenville gneiss. Sheared rock was also noted along the western 

 side of Moon mountain. The maximum throw of this fault appears 

 to have been no less than 600 feet. 



County House mountain fault. This fault is shown dotted on 

 the map because its presence is not altogether certain. The steep 

 eastern front of the County House-Kelm mountain masses and 

 the fact that the Grenville (much limestone) comes in rather 

 sharply against the bases of these mountains are the principal evi- 

 dences for the faulting. The Grenville here, contrary to the usual 

 thing, either dips aw^ay from the fault or its strike is at high angles 

 against the fault. 



Kelm mountain fault. This is a good example of a fault wholly 

 within a mass of very homogeneous, igneous rock. The fault scarp 

 is much more in evidence than the contour lines suggest, and there 

 is a rather distinct eastward downtilt of the block. Shear zones 

 were noted in two or three places. The displacement is as much 

 as 300 feet. 



Millington brook fault. This fault lies along the eastern side 

 of the valley of Millington brook and strikes north-northwest by 

 south-southeast. Wliere it crosses the road, i '4 miles south- 

 west of Kelm pond, the rocks are considerably sheared and 

 brecciated. On the western side of the small mountain ( 130J 

 feet), near the north end of the fault, sheared rock was also noted. 

 The extension of this fault southward to the western base of County 

 House mountain is somewhat doubtful though the relations of the 

 rock masses and the topography strongly suggest it. The throw 

 of this fault is probably not over a few hundred feet. 



