The sedimentary rock shown here
was deposited in layers from a huge old inland ocean. The rock consists
of sandstone and limestone. The sandstone is generally the top layers,
while the limestone is the bottom layers. Because of its marine origins,
fossils are often found in the limestone layers. Any strata (layer)
of rock that you find has a unique assemblage of fossils which act as a
way of identifying the general age of the rock layer. By knowing
the general age of the fossils inside the rock, scientists can translate
those figures into an estimate of the age of the layer. Strata containing
the same fossil assemblages are of the same time period, regardless of
where the columns were taken from. As with any unfolded column of
strata, the oldest layer of rock is on the bottom, while the youngest is
on top.