Chapter 17 Unraveling Geologic History

386 CHAPTER 17: UNRAVELING GEOLOGIC HISTORY
Niagara River
Lockport dolostone
Rochester shale
Irondequoit limestone
Reynales limestone
Thorold sandstone
Grimsby sandstone
Power Glen shale
Whirlpool sandstone
Queenston shale
Figure 17-1 The profile
of Niagara Falls shows
the oldest rocks are at the
bottom and the layers become
younger toward the
top. This illustrates the law of
superposition.
younger sediments can be deposited on top of them. Superposition
is the concept that unless rock layers have been
disturbed, each layer is older than the layer above it and
younger than the one below it. Therefore, when geologists assign
relative ages to rocks, the oldest rocks are on the bottom
and the layers become progressively younger toward the top.
Figure 17-1 is a profile of the rock layers at Niagara Falls.
Each rock layer is named for the location where it can be observed
and studied. For example, the hard cap rock at the top
that is responsible for the falls is the Lockport Dolostone. If
you want to observe this layer in the field, there is a bedrock
exposure near Lockport, New York. The first layer deposited
at Niagara Falls is the layer on the bottom, the Queenston
Shale. After this layer was deposited, sand was deposited
that became the Whirlpool Sandstone. One by one, each layer
was deposited before the layer above it.
Superposition does not always apply. In some places, folding
has overturned the rocks or faulting has pushed older layers
on top of younger layers. These exceptions to the law of
superposition will be explained in the next section. However,
if you are looking at rock layers or a diagram of rock layers,
unless you see evidence for overturning or faulting you should
assume that the law of superposition can be applied.The older
layers are those on the bottom. Exceptions are unusual.