1. LEQ: How do geologists and paleontologists determine the relative age of rocks? Part 2 Key Terms: Uniformitarianism, Original horizontality, cross- cutting relationships, unconformity

2. Other “Laws” of Geology Besides the “Law” of Superposition, geologists use three other “Laws of Geology” as tools for determining relative age of rock.

3. Uniformitarianism The present is the key to the past. – Geologic processes we observe happening on our planet today have occurred in Earth’s past, and will occur in Earth’s future.

4. Original Horizontality Sedimentary rock layers are initially deposited in horizontal layers.

5. Cross-Cutting Relationships The object that is being cut (or cut into) is older than the process that is doing the cutting. – A rock layer must be there before it can be cut. i.e. faulting – This relationship also applies to igneous intrusions The rock layer must be there before the magma can intrude into it.

6. Gaps in the Geologic Record The geologic record of sedimentary rock layers is not always continuous. Sometimes gaps appear in the geologic record. A gap in the geologic record of sedimentary rock layers is known as an unconformity. Click here to view the formation of an UnconformityClick here to view the formation of an Unconformity

7. Types of Unconformity Types of Unconformity:Types of Unconformity – Click on the above link to view the 3 types of unconformities discussed below. Disconformity –The boundary between two sequences of sediment. Nonconformity –The boundary between the igneous or metamorphic “basement” layers and the sedimentary layers. Angular Unconformity –The boundary between tilted layers and the sedimentary layers.

8. Sequencing in Rock Layers Click here to observe the sequence.

9. The following activity lets you be the geologist. There are five different pictures of one rock sequence throughout time. Below each of the pictures is the letter of the process being illustrated. On your notes sheet, print the order of the sequence from oldest to youngest using the letters of the process of each picture.

10. DCADB C Observe and try your best to determine the sequence.

11. 1 DCADB C 2 3 4 5 Check your answer!

12. 1 DCADB C 2 3 4 5 Which is the only number that shows an unconformity? 5