360 C. A. COTTON 



from the topography, for the main Hnes of the relief were deter- 

 mined by an orogenic uphf t which took place after the deposition of 

 the conglomerate/ 



Of the three possible ways in which differential uplift might take 

 place, namely, folding, warping, and block-faulting, the first two 

 seem to be out of the question since the surface of the Grey Marl, 

 in the area where it is preserved, was not appreciably tilted by 

 the movement, and appears to have been neither elevated nor 

 depressed to any extent by it. There remains the hypothesis of 

 block-faulting with the restriction that the uplifted block alone 

 moved. 



Significance of faults in the conglomerate. — Movement of the 

 nature of block-faulting, giving rise to mountains of the Basin 

 Range type, usually takes place along normal faults,^ and, for a 

 long period after the main faulting, the formation of small normal 

 faults continues, the younger faults so formed dislocating the fan 

 deposits resulting from the erosion of the earlier fault scarp. ^ 

 The occurrence of numerous small faults dislocating the Great 

 Marlborough Conglomerate and the underlying series is, therefore, 

 of some importance. In the Mead and Dee gorges, and in the 

 gorge of the Limburne, a stream between the Mead and the Dee, 

 the conglomerate, wherever examined, was found to be traversed by 

 a number of faults with small throw. The fault planes are now 

 in some cases nearly horizontal, and in others highly inclined, and 

 the downthrow is fairly often found to be on the opposite side from 

 the hade of the fault plane. If, however, the hade be measured 

 from a normal to the plane of bedding, instead of from the vertical, 



^ C. A. Cotton, "Physiography of the Middle Clarence Valley," Geographical 

 Journal, XLII (19 13), 228. 



= The advocates of lateral crustal extension and of some phase of crustal com- 

 pression as the active agency in the formation of block mountains appear to agree 

 that in most though not all cases the boundary faults hade toward the downthrow 

 direction and that the planes of later movements on approximately the same lines 

 are similarly normal fault planes. 



3 See G. K. Gilbert, U.S. Geol. Surv., 2d Ann. Rep., p. 200, 1882; and Mono- 

 graph I, pp. 340-57, 1890; I. C. Russell, U.S. Geol. Surv., 4lh Ann. Rep., pp. 451, 453, 

 1884; and Monograph XI, pp. 274-83, PI. XXVIIIB, 1885; W. M.- Davis, Btdl. 

 Mas. Conip. Zool., XLII, 129-73, 1903; G. D. Louderback, Bull. Geol. Soc. Am., 

 XV, 322, 1904; A. C. Lawson, Bull. Seismol. Soc. Am., II, 193-200, 1912. 



