map ofsedimentary rock outcrops in the US Sedimentary rocks develop from sediments such as mud or sand, usually in the bottoms of lakes or the ocean but sometimes on land, that turn into rock.

On the map at the right you can see that most folks in the USA live where sedimentary rocks outcrop, if there are any rocks outcropping at all. This is good news for most of us because, on the average, sedimentary rocks are easier to identify than other rock types, plus  sometimes they bear fossils in them! When we find fossils, we're better able to reconstruct what conditions must have prevailed when our rocks were deposited.

Sedimentary rocks in northern Alabama, photo by Bruce J. "Sleazeweazel" Morgan of Alabama

An important feature of many sedimentary rocks is that they are layered, as shown by the sandstone at the left. Maybe millions of years ago a flood caused a layer of sand to be deposited. Then maybe a few years or many years later another flood left another layer of sand. This process continued for millions of years, the lower layers gradually getting squeezed more and more by the heavy deposits above them. When you put enormous pressures on sand for a long time, the sand grains fuse together, forming sandstone. The process of turning loose material into rock is known as lithification.

Particle size often is important in determining your rock's identity.

(">"= "larger than"; "<"="smaller than")

Boulders    > 200/250 mm (> 7.9 inches) *
Cobble    63/75 – 200/250 mm (2.5 - 7.9 inches)*
Gravel    2 – 63/75 mm (0.08 - 2.5 inches)*
Sand   0,063 – 2 mm
Silt   0,002 – 0,063 mm
Clay    < 0,002 mm

* particle size for boulders, cobbles and gravel slightly larger in European system

Layering in sedimentary rocks isn't always as obvious as in the picture. Limestone created by the regularly proceeding chemical precipitation of calcium carbonate onto the ocean floor may not show apparent layering at all. Layers that do show up often are much thicker than those shown, and often much thinner. Most layers of sedimentary rocks are distorted somehow -- squeezed into curves or tilted at crazy angles, for instance. You can see that the layers in the picture dip downward toward the left.


Here are the most commonly encountered sedimentary rock types:


concretionsHave you ever mixed oil and water, and noticed how droplets of oil joined together to form larger and larger droplets? Something a little like that happens when sand, mud and general muck is deposited, then sits around for millions of years turning to rock. Sometimes inside the sand, mud or muck certain chemicals come together like oil droplets in water, and as the sand, mud or muck turns to rock, so do the chemical concentrations. The result may be that the chemical concentrations become concretions embedded in the rock, such as those shown above exposed in a rock wall.

What is a concretion?

The word "concretion" is a general term used rather loosely for several more or less rounded bodies mainly of non-living origin, occurring in all types and ages of sedimentary rocks. They are different in composition from the surrounding rock material and are generally harder and thus more resistant to weathering, or wearing away, than the surrounding rock.

Concretions are not at all rare in sedimentary rocks. They can form in various ways and not all geologists are in agreement about how they come about, but the way I've just explained them is at least recognized by most as sometimes being a likely scenario.

Concretions can be spherical, flattened, or irregular with lobes arranged in every conceivable way. They can be very small to very large. Spherical ones can be five feet or more in diameter and elongated ones 20 feet or more. Most are measurable in inches, however.

Concretions can have clearly defined outlines or their boundaries can be indistinct. Their surfaces can be rough or smooth. They can be homogeneous, or horizontally layered, or composed of concentric bands. All concretions appear to have grown outward from a center.

One important kind of concretion is the chert concretion, which forms in limestone and dolomite. Limestone and dolomite can dissolve away leaving chert concretions in place because chert, being silica dioxide, doesn't dissolve the way limestone and dolomite do.


Especially in roadcuts through sedimentary rocks you can often see faults -- fractures in the earth's crust where one side of the fracture has been displaced relative to the other side. The picture below shows a fault cutting across sedimentary limestone rock exposed in a roadcut. Roadcuts are great places to see faults.


In the picture the fault line runs from one side of the picture to the other. Notice that layers below the fault line seem to lie horizontally but above the line layers not only dip steeply to the left but also have been twisted relative to the lower ones. Also observe the highly fractured, crumbly nature of the stone at the fault line itself. This is where rock shattered and crumbled as the two rock face ground against one another.


geode with quartz crystalsGeodes are a little like concretions, in that they are formed in place, surrounded by rock, and sometimes the rock around them erodes away, leaving them lying about. However, unlike concretions, geodes are hollow, and typically are lined inside with inwardly projecting quartz crystals, like the one shown at the left. A typical geode is about baseball size, but they can range from an inch or less to much more than a foot in diameter. Some rare geodes produce crystals of minerals other than quartz, such as gypsum and calcite.

There is uncertainty as to how geodes are formed.

Don't forget that fossils are often found in sedimentary rocks. Check out our fossil page.