1. N. Lindsley-Griffin, 1999 Proterozoic Quaternary Cretaceous The Geologic Column [Fig. 3.8, p. 64]

2. © Houghton Mifflin 1998. All rights reserved Dating the Geologic Time Scale Sedimentary rocks not easily dated radiometrically Radiometric ages added to geologic time scale by: 1. Cross-cutting relationships 2. Bracketing

3. Relative Age - Principles N. Lindsley-Griffin, 1999 Unconformities: Gaps in the rock record - recognized by an erosional surface Signifies a major break in deposition Columbia River Gorge, OR-WA

4. Relative Age - Principles © Houghton Mifflin 1998. All rights reserved Horizontal sedimentary rocks Dike cuts strata - dike is younger Fault cuts dike and strata - fault is youngest Original horizontality Superposition Cross-cutting relationships Youngest sedimentary rock Oldest sedimentary rock

5. © Houghton Mifflin 1998. All rights reserved Dating the Geologic Time Scale Radiometric age of intrusion Y is 350 m.y. Radiometric age of intrusion X is 400 m.y. Devonian strata (B) must be older than 350 m.y. and younger than 400 m.y. Mississippian strata are younger than 350 m.y. Silurian strata are older than both Devonian and 400 m.y. Radiometric dating and cross-cutting relationships bracket ages of sedimentary rocks

6. Angular unconformity - younger layers over tilted layers - Grand Canyon, AZ N. Lindsley-Griffin, 1998

7. Angular unconformity - younger layers over tilted layers - Grand Canyon, AZ N. Lindsley-Griffin, 1998

8. Disconformity Angular Unconformity Time gap without angular relationshipTime gap after folding, faulting N. Lindsley-Griffin, 1998

9. Disconformity Angular Unconformity Time gap without angular relationshipTime gap after folding, faulting N. Lindsley-Griffin, 1998

10. “Baked zone” - soil formed on older lava flow before being covered and cooked by a new lava flow - Disconformity N. Lindsley-Griffin, 1998

11. “Baked zone” - soil formed on older lava flow before being covered and cooked by a new lava flow - Disconformity N. Lindsley-Griffin, 1998