418 J. W. Powell—Types of Orographic Structure. 
corrasion due to antecedent or superimposed drainage; that is, 
the corrasion of streams that head in remote regions and 
pass through these uplifts either longitudinally, transversely or 
obliquely, as in the case of Simple Anticlinals.* 
There are other modifications which sometimes greatly ob- 
scure the general topographic outline due to consequent drain- 
age, i. e., the local drainage which is due to the upheaval itself 
and which produces interesting 
CONCOMITANT Forms. 
1. Subsidiary Plateans.—Sometimes the streams which head 
near the axis of such an upheaval, as they meander to the 
flanks, excavate valleys and divide the great block, which is a 
plateau in general outline, into minor plateaus which are sepa- 
rated by intervening but elevated valleys. This is especially 
the case where the streams in their upper courses follow for 
some distance the strike of the beds before turning to cross the 
more or less abrupt lines of maximum flexure. etimes 
these streams run in deep gorges; in such cases the plateaus 
are bounded by cafions. 
2. Projecting Ridges——When these consequent streams start- 
eads gorges are formed, with towering pnphisneel In 
such cases an irregular line of crags and peaks will be found 
5. Interrupted Monoclinal Ridges.—On the flanks of these up- 
heavals, but farther from the axis than the flanking peaks, mo- 
noclinal ridges are often found sometimes broken by gaps 
* For an explanation of what is meant by antecedent and superimposed drain- 
age, the reader is referred to the Report on the Exploration of the Colorado River 
fe 160, et seg. 
