Weed and Pirsson — Highwood Mis. Laccoliths. 3 



The Shonkin Sag Laccolith. 



Where the Shonkin Sag valley debouches into that of 

 Arrow River it has a wide and open character with walls of 

 sloping debris or deeply carved Cretaceous strata. It is cut by 

 several small streams of excellent water which burst out as 

 springs under the base of Square Butte and find their way 

 into Arrow River. Through the middle of the level valley 

 floor wander the alkaline waters of Flat Creek with feeble, 

 sluggish current, dried away in summer to standing pools. It 

 is crossed by the stage road from Benton to Lewistown, and 

 on the open flat stand the buildings of the relay stage station. 



Proceeding from here up the valley in a westerly direction^ 

 at first the heavy mass of Square Butte above and to the left 

 dominates the scene, and in the sandstones upon which it rests 

 are seen intruded sheets of black basaltic rock. About a mile 

 or so to the west appears on the left an intrusive mass of dark 

 colored shonkinite which rises in a cliff wall over a hundred 

 feet in height, the talus from whose foot slopes down to the 

 valley floor. This wall is not more than a few hundred yards 

 in length and the intrusion seems to be clearly a separate one 

 from that of Square Butte and the Shonkin Sag laccolith. 

 Where its contact is seen, it cuts abruptly through the light 

 colored sandstones. It has a quite distinct columnar structure 

 and although evidently not a typical or symmetrical laccolith, 

 so far as one can judge from the exposures seen, it is clearly a 

 small intrusion of laccolithic character. 



On the opposite, the northern and eastern, side of the valley 

 several dikes of basaltic rock are seen cutting the sandstone 

 wall ; then an intruded sheet appears in the sediments above 

 the talus slopes. This persists, and following it a turn in the 

 valley reveals the front wall face of the Shonkin Sag lacco- 

 lith. From the opposite side of the valley, whose width is 

 here sufficient, an excellent view of it is obtained, and it is 

 seen as a long columnar cliff stretching for a distance of about 

 one mile, and with a quite even and regular face, interrupted 

 at about the middle by a deep canyon-like gulch which cuts 

 down through the whole thickness of the cliff at this point. 

 The columnar structure is quite pronounced, the polygonal 

 columns having a diameter of a couple of feet or more, and in 

 length, that is in height, they were estimated to be over one 

 hundred feet, which measures the thickness of the laccolith 

 along the cliff front. Below, all along the front, the laccolith 

 is seen resting on the light brown Cretaceous sandstones which 

 form the floor upon which it lies. They are quite horizontal 

 and the regular even level of this floor is very noticeable. 

 From the foot of the cliff a great talus descends down to the 



