Laboratory Manual for Introductory Geology 4e

13.7 When Streams Don’t Seem
to Follow the Rules
Most stream erosion and deposition follows the principles you have deduced in this
chapter, but there are some notable exceptions. Actually, these streams aren’t violating
any rules; they are following them to the letter, but their situations are more
complex than the basic ones we have examined.
Consider, for example, the Susquehanna River as it flows across the Pennsylvania
landscape shown in FIGURE 13.20. This area is part of the Valley and Ridge Province
of the Appalachian Mountains and is characterized by elongate ridges and
valleys made of resistant and nonresistant rocks, respectively. Streams are the dominant
agent of erosion in the area. Then, consider the Green River, illustrated in
FIGURE 13.21, whose meanders are incised deeply into bedrock. In the following
two exercises, you will explore what sequence of events might have led to the apparently
unusual situations in which these rivers are now flowing.
FIGURE 13.20 The Susquehanna and Juniata rivers cut across the Appalachian Valley and Ridge Province in Pennsylvania.
77°15’W
77°0’W
333
1500
0 5 10 miles
0 5 235
10 km
22
Thompsontown
322
17
L o c k R i d g
1000
500
R a c c o o n R i d g
Cocolamus Cr
Contour interval = 100 feet
e
e
T u r k e y R i d g e
W i l d c a
17
B u ff a l o
t R i d g e
1000
M o u
34
500
n t a i n
N
1000
Liverpool
JUNIATA CO
PERRY CO
11
15
500
147
Millersburg
500
M A H A N T A N G O
B E R R Y
Wiconisco Cr
25
1000
Berrysburg
209
Elizabethville
1000
225
Pillow
M O U N T A I N
M O U N T A I N
76°45’W
Gratz
1000
1500
Lykens
40°30’N
850
849
Newport
Little Buffalo Cr
New
Bloomfield
274
34
M a h a n o y R i d g e
R a t t l e s n a k e R i d g e
500
Little Juniata Cr
P i n e R
i d g e
500
849
Duncannon
274
500
Juniata River
H a l f F a
850
500
C o v e M o u n t a i n
l l s M t
n
147
1000
PERRY CO
500
DAUPHIN CO
Dauphin
22
322
11 15
Marysville
Halifax
SUSQUEHANNA
P E T E R S
Clark Cr
S E C O N D
Powells Cr
1000
1000
500
M O U N T A I N
1000
Fishing Cr
325
1000
1500
S t o n y M o u n t a i n
S h a r p
Stony Cr
M O U N T A I N
443
B L U E M O U N T A I N
M o u n t a i
81
39
22
DeHart Reservoir
n
Beaver Cr
L i t t
1000
l e
B L U E
34
M o u n t a i n
PERRY CO
CUMBERLAND CO
1000
500
M O U N T A I N
944
81
RIVER
Harrisburg
83