Sedimentary Rocks Explained

As we have discussed in the rock cycle overview, there are three different rock types: metamorphic, igneous and sedimentary. We see all of these rocks exposed in one place or another at the Earth's surface - even if they were formed many kilometers below. For example, most metamorphic rocks were formed many kilometers below ground level, but millions of years of erosion of the rocks above eventually uncover these once hidden rocks. Sedimentary rocks, however, form at the Earth's surface.

One of the basic concepts of sedimentary rocks, which is pretty much common sense when you think about it, is that most of them, when they were being formed, started out flat. Think of it this way: if you took a handful of dirt and dropped it in a large bowl of water, the dirt would eventually settle out flat onto the bottom of the bowl. The same happens in nature, as water, loaded with mud, silt and sand (sediment) enters a lake or an ocean. The dirt will enter the ocean and settle flat on the bottom.

Rocks that are deposited by simple sedimentation are flat-lying. This is what they would like like beneath the surface of the Earth.

Taking this one step further (going back to our "dirt in a bowl" example) let's say you dropped red-colored dirt into the bowl first. Let this dirt settle to the bottom, then drop in some blue dirt. Again, after this settles, drop in some green dirt. If we were able to slice these layers of dirt in half and look at the dirt from the side, we would see the red dirt on the bottom, followed by the blue dirt above, and the green dirt on top. What does this tell us?

Again, common sense says the red dirt "happened" first, and is the oldest event, followed by the blue dirt. Finally, the green dirt "happened" last, and is therefore the "youngest". Again, same thing as in nature. When you look at the flat-lying group of rocks that you see along the road, the ones at your feet happened first, while the ones at your head are the youngest, and happened last in this sequence. This is known in geology terms as The Law of Superposition.

When you cut through flat-lying rocks, they can be seen from the side. This has happened in the Grand Canyon. The further down you cut into the rokcs, the older and older the rocks which become exposed.

So, given what we just told you, why is it that all rocks that we see aren't lying perfectly flat? (You should be asking this if you live in western Virginia or along the Hudson River in New York State.) Well, there are lots of possible answers depending on where you live, but we'll keep it simple. Remember earlier when we said that the solid earth is actually in constant motion? Well, when parts of the crust run into each other (or pull apart from each other), it causes solid rock to twist, break, fold and tilt. Sometimes, so much sand, silt and clay piles up in one area that part of it breaks off and slides downhill, sort of like a landslide. This is happening today in offshore Louisiana, where the Mississippi River is depositing hundreds of tons of sand, silt and clay per year into the ocean.

How do scientists describe the rocks they see? Click here to learn about the science of stratigraphy.

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