THE TWIN LAKES GLACIATED AREA 



3°; 



brings this out. Three prominent series of joints intersect the granite. 

 The direction of ice-movement is toward the east. One series of 

 joints has a dip of 70 E., and so runs almost at right angles to the 

 direction of ice-movement, and gives easy conditions for plucking. 

 Two other series of joints dip respectively steeply to the south and 

 more gently to the north, while the blocks into which they divide 

 the rocks pitch to the west. The result on the form of the ledge is 

 shown in the figure in which the glaciated surface rises to the east, 

 evidently determined by the jointing. When a ledge cut out from 

 a rock jointed in this manner is seen end on, it usually shows a 

 composite profile and not a simple glaciated curve, the profile being 



r- 





Fig. 7. — Glaciated ledges on south side of Lake Creek. 



clearly controlled by the jointing. The connection between joint- 

 ing and glaciated surface in such instances means that plucking has 

 played a large part in glacial erosion over the surface, as well as on 

 the lee side of the boss and that a process of pluck and heal has been 

 going on, the ice breaking out great blocks along planes determined 

 by jointing, and then rounding off the broken edges by glacial abra- 

 sion. This form of ledge surface is not a matter of occasional out- 

 crops; in the middle part of the valley the whole valley sides for a 

 distance of hundreds of feet vertically, and for considerable distances 

 along the valley, shows this intimate relation between joints and form 

 of the rock outcrops. On the south side of the valley, on both sides 

 of Crystal Lake Gulch, glaciated rock outcrops (Fig. 7.) rise from 

 the stream to the upper limit of glaciation, and all show glaciated 

 stoss sides rising at an angle of 30 toward the east, and ending in a 

 lee slope plunging steeply to the east. The two structures are deter- 

 mined by the line of jointing in the granite. Farther up the valley, 



