LAKE OUACHITA CONGLOMERATE ROCK EarthCache

This Earthcache is located on the Caddo Bend trail at Lake Ouachita State Park. 

In order to claim this Earthcache as a find you must answer the questions below and send me an email with the answers.  Do not post your answers in your online log.  Do not take photos that would give away the answers and post them to the cache page.

In order to see this Conglomerate rock you will need to go to the coordinates listed above.  You will be able to see the rock from that point or by water. You might want bring binoculars to get a close up view of the rock.

When talking with one of the State Park Interpreters we learned this the only location on Lake Ouachita you will find these Conglomerate rocks.

Notice the large gray masses of rock sitting on the hill above the tilted rock at lake level. If the water is down you can see another one at the water line. These Conglomerate rocks are one of the many wonders about Lake Ouachita since there are many unknown things that still baffle geologist about the uplifting that occurred causing the Ouachita Mountains to form during the collision of continents called the Ouachita orogeny.

These masses are called exotics because the pieces of rock that make up the mass were broken off the shelf and carried down the slope of the continental shelf to their current position. The exact mechanism that brought the material down the slope is unknown, however we can postulate that there was possibly an earthquake that broke the rocks off the shelf and caused them to slide down the slope.

What is Conglomerate?

 

Conglomerate is a clastic sedimentary rock that contains large (greater than two millimeters in diameter) rounded clasts. The space between the clasts is generally filled with smaller particles and/or a chemical cement that binds the rock together. These gray rounded masses look very distinct from the underlying rocks.


 

What is the Composition of Conglomerate?

 

Conglomerate can have a variety of compositions. As a clastic sedimentary rock it can contain clasts of any rock material or weathering product that is washed downstream or down current. The rounded clasts of conglomerate can be mineral particles such as quartz or they can be sedimentary, metamorphic or igneous rock fragments. The matrix that binds the large clasts together can be a mixture of sand, mud and chemical cement. They weather and erode differently because they are a conglomerate made up of boulders, cobbles and pebbles of sedimentary rocks.


 

How Does Conglomerate Form?

 

Conglomerate forms where a sediment of rounded clasts at least two millimeters in diameter accumulates. It takes a strong water current to transport and shape particles this large. So the environment of deposition might be along a swiftly flowing stream or a beach with strong waves. There must also be a source of large-size sediment particles somewhere up current. The rounded shape of the clasts reveal that they were tumbled by running water or moving waves. Conglomerates often begins by being deposited as a sediment consisting mainly of pebble and cobble-size clasts. The finer-size sand and clay which fill the spaces between the larger clasts is often deposited later on top of the large clasts and then sifts down between them to fill the interstitial spaces. The deposition of a chemical cement then binds the sediment into a rock. 


In September, 2012, NASA's Mars rover Curiosity discovered an outcrop of conglomerate exposed on the surface of Mars. The rounded clasts within the conglomerate provide evidence that a stream or a beach had moved the rocks and tumbled them into rounded pebbles. This conglomerate was the most convincing evidence that water once flowed on the surface of Mars.