6i2 ROLLIN T. CHAMBERLIN 



belt, but as the map shows only small oblique faults, this supposi- 

 tion has little or no tangible support and need not be seriously 

 considered here, as the movements already discussed would still 

 be needed to explain the oblique faults. If any underlying longi- 

 tudinal fault is to be postulated, a group of such faults with dis- 

 tributive action would best fit the case. 



On the glacier principle, the southern portion of the district, 

 moving eastward relative to the northern portion, should cause 

 repeated crevasse-like fractures to open along a belt where the 

 strain from the differential motion was greatest. As the south 

 side of the belt moved eastward with respect to the northern side, 

 tension would be developed along N.W.-S.E. lines obliquely 

 across the disturbed zone. The result would be a large number of 

 fracture hnes at right angles to the direction of tension, or running 

 N.E. and S.W., as in the upper part of Figure 2. The oblique 

 crevasses in a glacier point upstream as they extend toward the 

 middle of the glacier, or toward the more rapidly moving portion. 

 Hence in this fault problem the fractures should be inclined toward 

 the direction from which the relative motion came. This is 

 just what was observed in the faulted zone of the Lake Basin field. 

 It is then only necessary to suppose that the zone of yielding 

 between the differently moving areas was located where the faults 

 have developed. 



Near the eastern end of the fracture belt the downthrow side 

 of nearly all of the faults is toward the northwest. As these are 

 normal faults, most of the fault planes dip to the northwest. On 

 the other hand, near the western end of the belt the downthrow 

 side, and hence also the dip, of the majority of the fault planes is 

 toward the southeast, though the faulting is less regular at this 

 end than at the other. The downslipping in general has thus 

 been from the ends toward the middle of the belt, though the 

 generalization is not so well substantiated in the western half 

 flanking the Big Coulee-Hailstone dome as it is in the more regular 

 eastern half bordering the Big Horn uplift. The dome at either 

 extremity represents a relative upthrust which, if the result of 

 igneous activity beneath, necessarily adds a further element of 

 tension, thus favoring an increase in the number of fractures upon 



