STOP 3: Outcrop of Wissahickon showing large recumbent fold.
In the photo
you can follow a layer of quartzite in the Wissahickon that bends up and
back on itself. This evidence of deformation in the rock provides additional
clues about their history. Nearly all sediments are deposited so as
to form extensive, horizontal layers. When we see them in different
orientations we know that layers have been moved by geologic processes.
In this case the movement has tilted the layers to their current slant or
dip of about 45° and in the quartzite, it is possible to trace the layers
through areas where they have been folded. If you think about what might
do this to a layer of rock, you can probably develop an analogy -- a sheet
of paper. Hold two ends of the sheet and move your hands together.
As your fingers approach, the paper will bend, eventually taking on an appearance
not unlike the folded quartzite shown here.
The difference
of course is that if you took a layer of rock in your fingers, you wouldn't
be able to bend it. 
However, if the rock were heated and under considerable pressure already,
a small unevenness in the pressure can cause the rock to change its shape
and bend like the quartzite layers shown here. We can infer then that the
process that moved these sediments to depths of 20-25 km also acted in a way
to create the folds we see in some of the layers.
By the way, if you are at the
outcrop, you will see that the schist layers do not appear to be folded
in the same way as the quartzite. This should seem a little puzzling
as the rocks were all part of a package when the quartzite was folded.
The reason for the difference is in the quality of the minerals in the rock.
Unlike quartz, mica with its very platy grains tends to respond to uneven
stress by dissolving and regrowing so that its plates are aligned perpendicular
to the greatest pressure. This results in a more or less continuous
reorientation of the layers in the mica-rich schist and a loss of structure
that would show the folding. Quartz grains are more symmetrical and
so do not respond to uneven pressure in the same way. Quartz-rich layers
tend to preserve their integrity and bend or break when subjected to uneven
stress.
A small stream
illustrates the effects of erosion and deposition.