34 
PENNSYLVANIA ANORTHOSITES 
tation of the present drainage system. The East Branch of 
Brandywine Creek (see topographic map) is now flowing ap¬ 
proximately parallel to and within a half-mile of the northern 
border of the anorthosite. In early youth, the course of the 
stream may have been controlled by the line of least resistance, 
which was the northern' contact between the anorthosite and the 
quartz-monzonite; later, with the deepening of the channel, this 
line of contact continually shifted to the left (figure 2) parallel 
//oneyhrook Le 
Present Level 
Figure 2. Section at Barneston Figure 3. Section at Birdell 
In which 1, Quartz Monzonite; 2, Anorthosite; 3, Cambric Quartzite; 
4, Baltimore Gneiss 
to the channel, and along the slope of the valley. At the present 
time, the stream possesses a deeper and wider valley. Hence, the 
quartz-monzonite is even further removed laterally from the chan¬ 
nel, while the parallelism between the line of contact and the 
direction of the stream’s course is still pronounced. 
Again, along the southwest boundary of the anorthosite, the 
present course of the West Branch of Brandywine Creek sug¬ 
gests a somewhat similar history. The stream is seen to be flow¬ 
ing along the contact between the quartz-monzonite and the anor¬ 
thosite. In its infancy, this stream was probably working on 
quartz-monzonite which covered the anorthosite, and was making 
use of the fault plane which furnished a well-defined line of least 
resistance (figure 3). Later, by vertical cutting, it found the 
contact between quartz-monzonite and anorthosite. Eventually, 
with further deepening of its channel and broadening of its valley 
wall, the stream should lie well within the boundary of the anor¬ 
thosite, the quartz-monzonite should disappear on the south, while 
the fault-line should shift further and further away from the chan¬ 
nel, parallel to the stream valley. 
This initial economy of effort manifested in the early adjust¬ 
ment of these streams to their environment, has its analogue in the 
