RECESSION MORE RAPID THAN SHIFTING 21 1 



the advancing waters have an advantage on the headlands because of 

 their exposure, and a corresponding advantage in the bay heads because 

 the cliff there is lower. The shape of the shore tends to adapt itself by 

 a suitable curvature, so that these two advantages shall be in equilibrium. 

 Within limits, therefore, cliff-cutting may be entirely continuous along 

 a sinuous shoreline. Since the form of the shoreline at any one time is 

 a type of the contour lines on the surface of the cut terrace, in the case 

 in hand the terrace has a fluted surface with low ridges running seaward 

 along the line of retreat of each headland. 



It is especially to be noted that the rock surface of such a cut terrace 

 will be unweathered. The sediments laid on it, being derived partly 

 from the cliffs and partly from active streams, may be of the coarsest 

 sort and may be quite fresh even when feldspathic. Both these condi- 

 tions clearly distinguish this case from the advance of the sea over a 

 peneplain with its deeply disintegrated rocks. In the latter case the 

 sediments can be coarse only when the mantle rock of the peneplain 

 contains peculiarly resistant pebbles. 



APPLICATION OF CASE II TO THE AREA STUDIED 



In the area defined in the opening paragraph the contact of the Ar- 

 chean granite and Wyoming sandstone presents the features appropriate 

 to that phase of case II. in which denudation by cliff-cutting has cut 

 beneath the valley bottoms of the submerging land. Ignoring the sub- 

 sequent folding which made the Boulder arch, and supposing the east- 

 ward tilting due to mountain-making to be undone, the surface of the 

 Archean- Wyoming contact has a maximum relief of perhaps 500 feet. 

 Its undulations are of the smoothest sort, showing no slopes less than 3 

 miles long, with a maximum steepness of about 150 feet to the mile. 

 These long slopes are entirely unbroken by valleys in the Archean floor, 

 and the rocks of the latter are quite as fresh at the contact as at any 

 greater depth. These features indicate that wave erosion by the ad- 

 vancing Wyoming sea pared the entire surface, even the valley bottoms, 

 to a depth at least equal to the thickness of the zone of weathering ; 

 that in so doing the minor features of relief were erased and the major 

 features considerably reduced. Cliff-cutting was therefore substantially 

 continuous along the whole of the curved shoreline, and the sinking of 

 the land was slow enough to admit of a mature shoreline topography 

 during the stage recorded in the present outcrops. 



CASE III. RECESSION LESS RAPID THAN SHIFTING BY SUBMERGENCE 



Assume, as a third case, that the rate of migration is less than shifting 

 by submergence alone would produce. On this supposition the surface 



