386 The American Geologist. •^""«' ^^^^ 
But these inferences fail to take into consideration the im- 
portant change in topography which may accompany the advance 
of the sea. 
There are two factors which will control the surface of con- 
tact, namely: (1) subaerial erosion, (2) marine erosion. These 
factors may combine in any proportion, of which there are two 
hypothetical extremes: (1) rapid subsidence and minimum wave- 
erosion leaving the subaerial topography unaltered. (2) slow sub- 
sidence and the complete dominance of shore erosion destroying 
the relief of the submarine land surface. 
Neither of these extremes is considered by Mr. Fenneman. Of 
all possible relations between rate of subsidence and of wave 
erosion Mr. Fenneman discusses three hypothetical cases. 
(1) Cliff-recession due to wave erosion more rapid than shift- 
ing of coast-line due to submergences; in this case a gentler sea- 
ward inclination than that of the slope produced by subaerial 
erosion would result, or if the original slope is broken by valleys 
and ridges the latter will be truncated. Whether this truncation 
reaches the valley bottoms will depend upon the rate of subsidence. 
Cliff recession will take place against accumulating difficulties due 
to increasing bight and length of cliff and absence of bays. 
Case 2. Cliff recession progressively retarded until its rate 
equals that of the shifting of the shoreline due to submergence; 
In this case, at first a gentler seaward slope than that produced 
by subaerial erosion would be formed as in case 1. Then, when 
•the rate of recession equaled that of shifting, the slope of marine 
denudation would be equal to the slope of subaerial condition and 
the receding cliff will have a constant hight. This hight will be 
such as to allow recession at the same rate at which the shoreline 
would move landward by submergence alone. 
The land surface will thus be pared down uniformly. The de- 
nudation may or may not cut below the valleys of subaerial 
erosion. If it does cut below the valley bottoms the entire surface of 
contact will be fresh rock. If the paring is not below the valley 
bottoms, the weathered rock of the valley bottoms will be pre- 
served on the lower side of the contact surface, while the rock 
surface of the cut planes which replace the ridges will be fresh and 
the sediments laid down upon it, derived from cliff cutting and 
active streams will be coarse and fresh. 
These two facts distinguish this case from the advance of the 
sea without cliff cutting over a peneplain with its mantle-rock of fine 
material and its deeply weathered underlying rock. 
Mr. Fenneman applies this case to the area studied: The 
Archaean-Wyoming contact has, he states, a maximum relief of 
perhaps 500 feet. Its undulations are extremely smooth, no slope 
being less than 3 miles long and the maximum steepness 150 feet 
to the mile. These slopes are unbroken by valleys, and the 
Archaean rocks, as stated earlier, are of uniform freshness. These 
