476 0. W, HAYES — COOSA VALLEY IN GEORGIA AND ALABAMA. 



Although the residual surface material usually conceals the fault 

 plane, artificial cuttings at some points reveal the exact contact between 

 the overthrust and underthrust strata. As stated above, the green silici- 

 ous shales of the Coosa valley are always highly contorted, but much 

 more so near the fault than elsewhere. The original bedding is some- 

 times quite obliterated and the rock is in the forni of small lenticular 

 masses with slickensided surfaces. The rocks above the fault plane are 

 usually much less disturbed than those below. In all cases where the 

 exact contact could be seen the rocks above the fault plane were the 

 Rome sandstones and their greater rigidity may in a measure account 

 for their less disturbance. 



Just south of the Etowah river in Rome the fault plane is well ex- 

 posed. The rocks are crushed and deeply weathered, but the bedding, 

 so far as it can be determined, is approximately parallel above and below 

 the fault plane. A stratum of obscurely bedded red and yellow clay 8 

 or 10 feet in thickness marks the plane on which the maximum motion 

 occurred. It contains fragments of the overthrust and underthrust rocks 

 and is evidently a finely comminuted and deeply weathered fault breccia. 

 A large amount of motion has evidently taken place within the strata 

 on both sides of the main thrust plane, and every bedding plane for some 

 distance above and a considerably greater distance below shows sicken- 

 siding. 



In a roadcut about six miles southwest of Rome the fault plane is 

 shown even more clearly than in the case above described. It dips 

 southeastward about 12°, although in the 20 feet observed the angle 

 varies from 10° to 15°. The thrust-plane is marked by a bed 2 or 3 feet 

 in thickness of reddish clay containing many small angular fragments 

 of the adjacent rocks. Above this fault breccia the sandstone is only 

 slightly disturbed, while below the green silicious shale has almost en- 

 tirely lost its bedding. 



Relation of minor and major Thrusts. — It will be seen from the accom- 

 panying map that while the minor thrust-faults have a nearl}'- north- 

 and-south trend they do not cross the Coosa fault, but in most cases die 

 out in the -band of Cambrian shale and sandstone upon its southeastern 

 side. At two points the Coosa fault appears to be intersected and offset, 

 but in neither case does the intersecting fault belong clearly to the class 

 of minor thrusts above described. It seems scarcely possible that if 

 major and minor thrusts were developed at the same time the latter 

 should not in some cases at least intersect the former ; also if the minor 

 thrusts had been developed subsequent to the major thrusts they would 

 have intersected the latter even more frequently ; hence it seems a fair 

 inference that the two types of faulting belong to distinct periods of or- 



