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Determining the Origin of a Sedimentary Rock

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  • 1:11 Stratification
  • 2:22 Graded Bedding
  • 3:16 Cross-Bedding
  • 4:20 Ripple Marks
  • 5:08 Mud Cracks
  • 5:59 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Sedimentary rocks contain features that indicate the kind of environment from which they originated. Learn about sedimentary structures, such as graded bedding, cross-bedding, ripple marks and mud cracks, and how they provide clues to the rocks origin in this lesson.

The Origin of Sedimentary Rock

With a little bit of sleuthing, you can determine the origin of sedimentary rocks. Of course, if you are going to be a rock detective, then you will need to be able to pick up on the clues sedimentary rocks have to offer. In this lesson, you will discover how structures, such as graded bedding, cross-bedding, ripple marks and mud cracks, provide clues about the environment where the rocks formed.

Sedimentary rocks preserve a record of history because of the way they form. Sedimentary rocks are comprised of bits of pre-existing rocks or minerals and some even contain bits of organic material, such as plant and animal remains. These sediment grains get moved around by natural forces, such as water, ice and wind, and then find a nice quiet place to settle. Layers of sediment, called strata, build up over time and get compressed and cemented together to form rock. By studying the way these strata are laid out and what they are composed of, you can uncover clues about the origin of the rock.

Stratification

Stratification is the term used to describe the layering found in sedimentary rock. You can recall this term by noticing that the word starts with 'strat,' which is short for 'strata' and ends with the suffix 'fication' meaning 'making.' Therefore, stratification is literally the 'making of strata.' With stratification, you see that different types of sediment get deposited on top of each other, kind of like a cake with many layers. And, like your layer cake, your sedimentary rock can have varying colors from one layer to the next.

These changes in color provide clues about the rock. Specifically, it tells you that the sediments within those different colored layers contain different minerals, and therefore originated from different rocks or different locations. You can get more clues by studying these layers based on the shape of different sediment grains. For example, if you found rounded grains, then you could assume that this sediment arrived in its final resting place via water, which rolled and tumbled the particles, causing the edges to smooth.

Graded Bedding

You can also derive clues by looking at the patterns by which the sediments deposit. For example, as your detective work continues, you might notice graded bedding, which is a sediment deposit characterized by a detectable sorting of grain size from bottom to top. With graded bedding you will typically see large, coarse sediment grains deposited at the base of the layer and small, fine sediments near the top.

When this arrangement is seen, it's a clue that at the time the rock was formed, there was a current of water loaded with sediment that rushed into place. This turbid flow allowed the larger and heavier particles to settle out first, followed by the lighter particles as the flow lessened. Graded bedding can give you clues about the age of rocks within a sedimentary sequence because the larger and older grains are typically found at the base of the bed.

Cross-Bedding

Cross-bedding is another clue revealed by sedimentary rocks that help you understand the kind of environment in which the sedimentary rocks originated. Cross-bedding is defined as layering within a rock that does not run parallel to the original horizontal layers. It is as if some of the sediment decided it did not want to lie down flat, and instead it chose to be deposited on a slope or an angle. So, when you look at a sedimentary rock with cross-bedding you will see the normal horizontal layers that you would expect to see, but you will also see some lines of rock lying across these layers at an angle.

This is your clue that this rock originated in an environment such as a beach, river or sand dunes. Why? Well, in these environments, forces, such as wind and water, cause the erosion and redepositing of sediment grains in piles. When these piles build to a certain point, they become unstable and the grains cascade downslope at an angle, giving the appearance of angled lines lying across the original strata.

Ripple Marks

Ripple marks might also be found in sedimentary rocks offering even more clues to the rocks' origin. Ripple marks are small wavy ridges of sediment caused by the action of waves or wind. These wavy ridges remain after the sediment hardens into rock, preserving the ripples.

If ripple marks are present then you might assume that the rock originated in a stream bed, lake or on a beach. For example, if the ripple marks look like waves, it is likely that they were created by the gentle lapping motion of waves that pushed sediment into this pattern. If the ripples form in one basic direction, this tells you that the rock structure was created by a water or wind current that was flowing downstream.

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