What Stories Do Rocks Tell?
ES2903  The Principle of Original Horizontality

Limestone layers form in flat layers at the bottom of a shallow sea. These gray layers of limestone are exposed in the Mojave Desert, pointing up at an angle from the flat desert floor. What story does this rock tell about the past? Which layers do you think are the oldest?

  !   Examine the photograph to see the tilted layers of limestone. Try to imagine the processes responsible for tilting sedimentary layers. Click the photograph to see an animation.

USGS/Jennifer Loomis, TERC
Tilted limestone beds in the Mojave Desert, California.

The principle of original horizontality states that most sedimentary layers of rock are deposited in a horizontal position.

5. How might layers that were once horizontal come to be tilted like this?


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