Principles of Geological History

Superposition Oldest sediment at bottom Youngest at top Image source: clarkscience8.weebly.com

Horizontality Gravity pulls things down Sediments settle evenly, so… Layers are initially horizontal What can happen later: Folding Faulting Tilting Image source: en.wikibooks.org

Lateral continuity Image sources: openclipart.org (below) wordwomanpartialellipsisofthesun.blogspot.com (right)

All 3 principles:  superposition  horizontality  lateral continuity Image source: coloringpanda.com 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.

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

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.

Which of these sequences shows the oldest to the youngest?

Igneous Intrusion

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.