7. The Law of Superposition states A. if there is a break in the rock layer, it is called an unconformity.
B. radioactive isotopes decay.
c. age is measured in comparison with another object.
D. each sedimentary rock unit is older than the unit above it.

Crinoids, also known as sea lilies, appear to be sea plants but are not plants at all. crinoids are an echinoderm species which uses anchoring structures called holdfasts to attach themselves to the sea floor. crinoids were most abundant in pennsylvania during the mississippian period, which ranged from 375 to 320 million years ago. fossils of the crinoids shows that during most of this time pennsylvania was that provided favorable conditions for crinoid growth. a) glacial terrain b) a dry and sandy plain c) mostly mountain ranges d) covered by warm, shallow seas