GEOLOGY OF LAKE PLEASANT QUADRANGLE 49 



dies out toward the headwaters of Colombe brook, while southward 

 along the base of the Groff-Cathead mountain masses the apparent 

 throw is from 500 to 800 feet. 



Dewey Creek fault. Only the southern end of this very prom- 

 inent fault comes within the map limits and joins the Colombe 

 Brook-Cathead Mountain fault at the base of Groff mountain. This 

 fault is fully 15 miles long and its topographic influence is very 

 pronounced, especially within the Stony Creek sheet where the 

 scarp rises abruptly from 400 to 800 feet. Its downthrow side is 

 on the west. Some crushed rock was noted along Dewey creek. 



Fault on east side of outlier. The character of the topography 

 and the very straight lines of outcrops of Precambric rocks at the 

 base of the steep slope mak^ the existence of a north-south fault 

 here almost certain, though the evidence is not conclusive. It ap- 

 pears to have a displacement of no less than 300 to 400 feet with 

 downthrow side on the west. 



Faults within the outlier. A minor fault sharply bounds the 

 mapped area of Little Falls dolomite on the west. The outcropping 

 dolomite north of the river forms a fairly distinct scarp. Where 

 the wedge of Lowville limestone comes against the dolomite, the 

 fault surface is plainly visible in a small quarry. At this quarry 

 the whole of the Potsdam and Theresa are faulted out so that the 

 throw is here about 100 feet. Northward the fault can not be 

 traced because of heavy drift deposits. 



The strong westward dip of the beds in the small area of Theresa 

 just east of the road is probably due to downfaulting of these beds 

 against the dolomite as shown on the map, though the relations 

 are not very clear. 



Speculator-Hamilton Mountain fault 



A very prominent fault has determined the steep eastern front of 

 the great Speculator-Hamilton-Swart mountain mass. It strikes 

 north-northeast along Jimmy creek ; the west side of Charley lake ; 

 and through Gilman lake. An important branch bears due north 

 from the notch just west of Round mountain and has determined the 

 steep scarp immediately east of the summits of Cutknife and Specu- 

 lator mountains. Evidences of shearing were noted along this 

 branch fault and also in the notch just west of Round mountain. 

 The downthrow side of this fault is on the east with a displacement 

 along Jimmy creek of fully looo to 1200 feet, while the combined 

 throws of the two branches of the fault east of Speculator mountain 

 are at least 1300 feet. 



