Comment: 13:28 - 14:47 (01:19)
Source: Annenberg/CPB Resources - Earth Revealed - 17. Sedimentary Rocks: The Key to Past Environments
Keywords: "sedimentary rock", "clastic sediment", "Principle of Original Horizontality", "sedimentary structure", bedding, cross-bedding, lakebed, "sea floor", wind, sand, dune, "mineral color", "mineral composition", "sand bar", "stream delta", geologist, stream, deposition, erosion, velocity, water
Our transcription: The "Law of Original Horizontality" states that most bedding initially forms in a horizontal orientation as material settles to a lake bed or the sea floor.
But in some cases, sloping layers of sedimentary rock build up; for example, wind can pile up sand as dunes.
As mineral grains of diverse color and composition are blown across the dune, discernible layering can develop, inclined at an angle parallel to the slope of the dune.
Such angled layers also develop in sand bars and stream deltas.
Geologists refer to this as "cross-bedding" because it cuts across the direction of ordinary horizontal bedding.
Sets of cross-beds often develop.
For example, at any given point in a stream, periods of deposition may alternate with periods of erosion as the velocity of the wind or water changes.
A set of cross-beds in a bar will be truncated by erosion, then covered by another set of cross-beds when deposition resumes.
Geology School Keywords