o-~ 
Planation Stream Piracy. 255 
Fig. 2. View of the flat-bottomed opening through which Upper Cut-off 
Creek enters onto the wide flood plain of Salt Creek at Phillips School. 
Salt Creek flood plain is confluent with this flat-bottomed opening, and 
is beyond the projecting spurs of upland shown at either side of the 
view. 
valley-flat of Salt Creek. The alignment of the valleys of the two streams, 
the presence of Cut-off Col between the valleys, the ending of the valley 
of Upper Cut-off Creek where the stream turns northward at Phillips 
School, the more sharply trenched condition of the upper valley. the under- 
sized stream in the broad gently-sloping lower valley, and the great semi- 
circular bend of the steep south bluff of the entrenched valley of Salt Creek 
show clearly that a once continuous stream has been divided and the 
upper portion caused to empty into Salt Creek valley several miles farther 
upstream than where the drainage formerly entered. This condition un- 
doubtedly resulted from the rather extraordinary widening of Salt Creek 
yalley in the Phillips School locality by lateral planation. 
Conditions Which Favored the Cut-off Piracy—-A number of conditions 
fayored the Cut-off planation piracy. The parent Cut-off Creek flowed 
almost parallel with Salt Creek in its westward direction near Fairfax. 
This parallelism was not an extraordinary thing in this small stream, as 
the southward turn of Salt Creek valley allowed it to come into Salt Creek 
ina normal manner. (The southward turn of Lower Cut-off Creek is rather 
exceptional, as it causes the parallelism of the two streams to continue a 
greater distance than it otherwise would; but a discussion of this condition 
is not essential to the present problem.) Lateral planation is a normal 
action taking place in valleys which have reached the mature stage. But 
the conditions in the vicinity of Fairfax are rather favorable for an un- 
usual amount of lateral planation. The valley here makes an abrupt bend 
somewhat greater than a right angle. Such a turn should normally cause 
the waters of the valley to impinge against the outside valley-wall, or in 
this case on the south bluff. It may be noticed that the valley is much 
