STOP 6: The rocks of stop 4 seen again, an illustration of the continuity of layering in the schist and the geologic terms, strike and dip.
The rock at
this pavement outcrop is similar to the rocks seen at stop 4. In fact,
you can see those rocks if you look up the hill and just a little to the right.
The connection between the two outcrops is real. Originally the
Wissahickon Schist was a sedimentary rock and like almost all sedimentary
rocks it originated as a deposit of sand, mud and clay spread as a horizontal
layer across a wide area. Each layer in the sediments can be distinguished
and if one had the proper tools could be traced across the entire basin in
which it was deposited. In the Rockies one can find rocks of the same
type and age in mountain ranges separated by several hundreds of miles.
Although the Wissahickon Schist has been altered by metamorphism
and deformation, the basic reality of its sedimentary origins remain. It
consists of a sequence of layers that can be traced from place to place. The
orientation of the layers in space is important to a geologist because he
or she would like to be able to predict where the layer is likely to appear
again. Because you look to the northeast to find the layer at stop 4,
we say the rocks strikes to the NE. That is the map direction
followed by the layer. To place the layer in the real world's three
dimensional space, not just the two dimensions of the map, we must also measure
its orientation in the vertical dimension. This is called the dip
of . It is the direction and angle at which a layers dips into the
earth. Here the layers dips at about 60° to the NW.