Geologic Time

3. in an undisturbed sedimentary rock sequence the oldest layer is at the bottom
5. one of the units of geologic time into which geologists divide eras
7. one of the three long units of geologic time between the Precambrian and the present
12. the process of determining the age of an object using the half-life of one or more radioactive isotopes
13. animal without a backbone
14. the age of something compared to something else
16. an even during which many types of living things become extinct at the same time

1. a record of geologic events and the evolution of life forms as shown in the rock and fossil records
2. a gap in the geologic rock record
4. an animal that lives part of its life on land and part of its life in water
6. the atoms of one element break down to form the atoms of another element
8. an animal with scaly skin and lays eggs that have tough, leathery shells
9. the number of years that have past since the rock has formed
10. an animal with a backbone
11. vertebrate that controls is own body temperature and feeds milk to its young
15. the preserved remains or traces of once living things