1. Principles of Geological History

2. Superposition Oldest sediment at bottom Youngest at top
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3. Horizontality Gravity pulls things down Sediments settle evenly, so…
Layers are initially horizontal
What can happen later:
Folding
Faulting
Tilting
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4. Lateral continuity Image sources: openclipart.org (below)
wordwomanpartialellipsisofthesun.blogspot.com (right)

5. All 3 principles:  superposition  horizontality  lateral continuity
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Eroded rock layers have been removed by rivers, glaciers, etc. Rocks are still horizontal and oldest rocks assumed to be at the bottom (could check using absolute dating methods). Because of similar rock types and fossils, we can assume lateral continuity – i.e., these rocks were once connected.

6. Horizontality Explain this sequence Image source: www.studyblue.com
Horizontal deposition, tilting, erosion, then further horizontal deposition on top of tilted layers.

7. Correlating rock strata Correlation = matching rocks or fossils of the same age between different locations.
Which fossils are oldest? Which are youngest? Why are there layers missing?
Image source: biobook.kuensting.org
Oldest = trilobites; youngest = fern (above coral and crinoid/ammonite layers). Layers missing due to erosion by glaciers or rivers.

8. Which of these sequences shows the oldest to the youngest?

9. Igneous Intrusion

10. Unconformity
Unconformities are gaps in the geologic record that may indicate episodes of crustal deformation, erosion, and sea level variations.
Unconformities represent times when deposition stopped, an interval of erosion removed some of the previously deposited rock, and finally deposition was resumed.