Law of Superposition
We already know that sedimentary rocks are formed in horizontal layers known as strata. The
law of superposition can help us determine which rocks are the oldest. This law simply states
that in s sequence of strata (rock layers), each layer of rock is older than the layer above it,
and younger than the layer below it.
Looking at the images above, we can infer from the Law of Superposition that the Cococino
sandstone is older than the Toroweap formation, but younger than the Hermit shale.
Another important law is the law of
crosscutting relationships. This basically
means that if a fault or another body of
rock cuts through another body of rock
then it must be younger in age than the
rock it cuts through. In the image opposite,
the dike of igneous (volcanic) rock cuts
thorough three layers of sedimentary rock,
and is thus younger than all three layers.
The Law of inclusions states that if a body
of rock (rock B) includes fragments of
another rock (rock A), this body of rock
must be younger than the fragments it
contains. The intruding rock (rock A) must
have been formed first to provide the
fragments.
Using the information on the previous page, complete the questions below.
1. List the strata in the image below in order of youngest rock to oldest rock.
2. List the strata in the image below in order of youngest rock to oldest rock.
3. List the strata in the image below in order of youngest rock to oldest rock.