Some river rocks are sedimentary, and some are not.
Let us explain.
Are River Rocks Sedimentary? (Answer Explained)
Most people have seen river rocks at one point or another.
However, many people may be unsure what type of rock they actually are.
The truth is, most river rocks that you see in their natural setting are a mix of many different kinds of rocks. Some of those rocks are sedimentary….and some are not.
What are Sedimentary Rocks?
Unlike metamorphic or igneous rocks, which are formed from great pressure and heat deep beneath the earth, sedimentary rocks form on or near the surface.
These rocks are created through a few geological processes such as weathering, erosion, precipitation, lithification, and dissolution.
Erosion and weathering slowly break down large rocks into smaller ones over time through wind and rain exposure.
Precipitation and lithification work to build new rocks.
Lithification is the process in which materials such as sand or clay are slowly turned into rocks via the weight of accumulating sediment.
Precipitation is the process whereby new stones are formed via chemicals that precipitate from water.
For instance, when a sea or lake dries up, the mineral deposits that are left behind can be used in this process.
When it comes to classifying sedimentary rocks there are two categories.
Detrital rock and chemical rock.
Detrital Sedimentary Rock
Detrital rock comes from the accumulation of rock fragments and other materials.
This detritus can be either organic or inorganic.
For instance, organic detrital rocks can form when material from plants or animals decays and leave behind biological matter that can become rock over time due to compression.
Coal is one such rock that fits this description and it is formed from the compressed remains of plants that died millions of years ago.
Inorganic detrital rocks are formed from the smaller pieces of other rocks that have either worn down from weathering or erosion.
This type is also often referred to as clastic sedimentary rocks.
Sandstone falls into this classification, as it is formed from layers of sandy sediment that were compressed and lithified over time.
Chemical Sedimentary Rock
This type of sedimentary rock can be found in many different locations, including caves, oceans, and even deserts.
To put this into perspective, most forms of limestone are formed from the compressed remains of shells and marine animals.
If limestone is found on land, that area was most likely underwater at one point in history.
Stalactites and stalagmites are also considered chemical sedimentary rocks, and they are formed by water passing through the bedrock and dripping down into the cave where these formations grow.
When the water passes through the bedrock it picks up minerals such as carbonate ions and calcium.
As this water settles in the cave and evaporates, it then leaves behind these minerals which can accumulate and form a stalactite or stalagmite.
How River Rocks are Made
Some river rocks fall into the category of sedimentary rocks due to how they are formed.
However, some river rocks can be metamorphic or igneous in origin.
As the moving water of the river moves over stones, they are eroded.
How quickly these rocks erode and become river rocks depends on how fast the water is moving.
A slowly moving river will require more time to turn stones into river rock than one moving at much faster speeds.
However, it is not this water that causes the rocks to weather as they do, but the smaller particles of rock, silt, and sediment that are carried in it.
These particles act as a source of abrasion which weathers and erodes the larger stones.
As a river moves faster, more of this material is moved to hasten the transformation into river rock.
As larger rocks are eroded, it’s common that pieces of them will break off and be carried away by the moving water.
Because of this, a constant stream of materials is delivered along the river, continuing the erosion and weathering processes on rocks further downstream.
How to Identify River Rocks
River rocks are a diverse group of stones that are grouped together due to having been worn down by the actions mentioned above.
However, there are a couple of specific types that you should know about.
Granite River Rocks
Granite river rocks are some of the most commonly found and they are formed from igneous rock.
This means that they originally began life as magma deep beneath the earth before cooling and crystalizing.
These river rocks often have coarse grains of minerals and a speckled or banded appearance.
They can also be found in a variety of colors, such as grey, white, russet, and green.
Schist is a metamorphic rock that formed deep in the earth.
These stones tend to be somewhat rectangular in shape and can be banded similar to granite.
Basalt is another igneous rock that formed from ancient lava flows.
This stone is usually dark grey or black due to either augite or pyroxene content.
This stone is prevalent in eastern Oregon, western Idaho, northern Nevada, and Washington.
This sedimentary rock is another that you should know.
Usually formed from the remains of marine animals and calcium carbonate, these stones are popular with fossil hunters as they often contain fossil deposits.
Shale is a sedimentary rock that forms from silt and mud.
It can be found in many colors, depending on the specific materials that went into its creation.
For instance, it can be black and grey, but in oxygen-rich environments, with a lot of iron, it can even take on a reddish-brown color.
This stone can often be found along flood plains or river deltas.
Quartz is another stone that you can find along some rivers.
It can be found in many different colors, but white sand can be a giveaway that the riverbed of a river is mainly composed of quartz.
Along the Llano River in Texas, you can even find blue quartz, along with clear varieties and other stones like topaz, agate, and garnet.
How to Find River Rocks
As the name might suggest, a great many river rocks can be found along the banks of rivers and in old riverbeds that have since dried up.
However, because river rock is a popular building tool, you can often even find it in some landscaping fixtures.
When looking around a river for these rocks, keep an eye out for gravel bars where rocks might accumulate or along freshly eroded cliffs along the banks.
These can all be great places to find high-quality river rock specimens.
A Fascinating Variety of Stones
While not all river rocks may have a sedimentary origin, their unique variety makes them one of the most interesting types of rocks you can find.
The next time you find yourself near a river, take a bit to walk along the bank and see what you can find.
You might be surprised by the variety of river rocks that exist in your area.