Conglomerate – Nature’s Concrete

Audra State Park is a heavily wooded area bisected by the Middle Fork River located in southwestern Barbour County and a portion of Upshur County. The coordinates bring you to a spot on a trail in the park. At this location there is a large example of a conglomerate. What we’d like to do here to compare and contrast this natural formation to man-made composite that we call concrete.

Conglomerate:

Conglomerate forms where sediments of rounded clasts at least two millimeters in diameter accumulate. It takes a strong water current to transport and produce a rounded shape on particles this large. For this reason, we are likely to find the clasts needed to form conglomerates along a swiftly flowing stream or a beach with strong waves. These conditions might only be met during times of extreme flow or wave action. However, it is during these times that much of the Earth's sediments are moved and deposited.

How Do Conglomerates Form?

To form a conglomerate, there must be a source of large-size sediment particles somewhere up current. The rounded shape of the clasts reveals that they were tumbled for some distance by running water or moving waves. These conditions are found in streams and standing water bodies in many parts of the Earth. Typically, the space between the gravel-size clasts is filled by a mixture composed of varying amounts of silt, sand, and clay, known as matrix.

Conglomerates often begin when a sediment consisting mainly of pebble- and cobble-size clasts is being deposited. The finer-size silt, 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. After compaction, the deposition of a chemical cement then binds the sediment into a rock.

Over the course of time, water draining through the clast and crushed limestone debris and calcium carbonate (lime) and spread it as cement in this deposit. This process creates a single larger rock known as a conglomerate. A material is considered a conglomerate if its clasts are larger than sand – greater than 2 mm in diameter.

At the above coordinates you will find a cemented rock sediment, known as conglomerate, that has slowly been exposed as the Middle Fork River has cut it valley into the landscape. Note that limestone is the primary source for conglomerate and concrete matrix!

The conglomerate rocks you see are very similar to modern day concrete. Both conglomerates and concrete are comprised of composite materials – clasts, which in concrete is called aggregate, that are held together by the much finer grain matrix.

Concrete:

Concrete is a man-made conglomerate of sorts comprised of an aggregate - rocks, stones and/or small gravel - and a matrix made up of Lime or calcium oxide, Silica, Alumina, Iron and Gypsum. The major ingredient of cement comes from limestone.

Located in the main park, at the stated reference point, N 39° 02.469 W 080° 03.996 there is an object that was once used in a gristmill, in the early 19th century. It is a good example of man-made concrete.

Conglomerate classes

The way conglomerates are classified is by texture and size of the clasts.

Texture:

Paraconglomerates consist of a matrix-supported rock that contains at least 15% sand-sized or smaller grains (less than 2 mm), the rest being larger grains of varying sizes. Paraconglomerates are also often unstratified and can contain more matrix than gravel clasts.

Orthoconglomerates consist of a clast-supported rock with less than 15% matrix of sand and finer particles. If the gravel clasts of a conglomerate are in contact with each other, it is called an orthoconglomerate.

The differences between paraconglomerates and orthoconglomerates reflect differences in how they are deposited. Paraconglomerates are commonly either glacial tills or debris flow deposits. Orthoconglomerates are typically associated with aqueous currents (watery solutions).

Size:

Conglomerates are also classified by the clast size of a majority of the clasts:

• Granule conglomerate 2–4 mm

• Pebble conglomerate 4–64 mm

• Cobble conglomerate 64–256 mm

• Boulder conglomerate >256 mm

Logging requirements:

The posted coordinates will bring you to an area on the trail with a large cube shaped rock. With the river at your back, you will see a much larger conglomerate rock to the right of it. Base the answers to the questions off this rock. (You should bring a ruler with mm scale)

1) For the conglomerate at the posted coordinates identify its texture from the description above.

2) Based on the information in the description, do you think that the conglomerate at the posted cords is the result of a glacial type deposits or aqueous currents?

3) For the conglomerate at the posted – how would you classify it by clast size as described above?

4) Optional: If you like to commemorate your visit with a photo there are numerous scenic locations in the park. Please do not include the conglomerate – to discourage internet earthcaching.

You may log the cache once you have visited the cache; however, if you fail to answer the questions in a timely way your log will deleted.