These giant boulders are known to geologists as concretions. Concretion comes from the Latin root word con “together” and cresco “to grow” Concretions usually form in sedimentary rocks.
The concretion begins to form when a small amount of minerals fill in the spaces between the grains of the sedimentary rocks—usually before the sedimentary rock has even finished hardening. Think of the minerals like a glue, holding grains of sand together. Very often, this mineral cement is harder than the sedimentary rock it surrounds. The concretions can be any shape, but most often they are round or slightly ovoid (egg shaped).
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There are two main types of concretions: a) concentric and b) pervasive. The concentric type grow in layers from the inside out, often forming spectacular patterns. These tend to look like the familiar gumballs of your youth. An example of this type would be the beautiful “Thunder Eggs” that are the official mineral of Oregon.
The pervasive type glues the sediments together from the outside in, or all at once. These have the appearance of a “snowball” (or other shape) made of tightly packed sandstone, siltstone, or shale. Those three rock types are the most likely to form concretions. This type most often includes fossils.
Concretions can form around a fossil initially, and smaller concretions are sometimes broken open to look for fossils.
Do not damage the boulders to collect fossils!
Best access and parking is on the S Frontage Road. To reach the location, take exit 152 off I-20/59 and head south to the frontage road.
The boulders originally come from the Bashi Formation of the Wilcox Group, which is exposed across most of the Gulf Coast coal region. In Mississippi, the lower units in the Wilcox Group include the silt, clay, sand, and lignite beds of the Nanafalia Formation and the Tuscahoma Formation. The Bashi Formation is a glauconite and fossil bearing sand, rich in fossiliferous concretions. It separates the lignite-bearing Tuscahoma from the overlying shale bearing Hatchetigbee Formation (Williamson, 1976; Bicker, 1969; Dockery, 1996).
The Bashi Formation was deposited during a marine transgression (rise in sea level) that was part of a larger sequence of cyclical changes in sea level. According to Danehy, Wilf, and Little, the contact between the Tuscahoma and Bashi Formations is marked by a regional unconformity that extends from western Alabama to the eastern region of Mississippi.
The local Bashi has two main divisions; a lowstand unit and the upper transgressive marine unit. Bashti divisions
The 3-meter thick lowstand unit is the lowest Bashi sediment in Mississippi. It contains many fossils not found elsewhere in the U.S. The upper transgression marine sedimentary rocks is made of sands containing the mineral glauconite and many fossiliferous boulders, in a layer slightly more than 1 meter in thickness.
Glauconite is considered an indicator of marine depostion with a slow rate of accumlation along a continental shelf.
The Mt. Barton section of the Wilcox Group is an outlier of the Tallahatta Cuesta, a prominent cuesta along the eastern flank of the Mississippi Embayment that extends from southern Alabama to north-central Mississippi. Mississippi Embayment
Cuesta is the term for ridges formed by gently tilted sedimentary rock layers. Cuesta comes from the Spanish word for slope. The slope of the two sides of the cuesta ridges are different. The escarpment where the rock layers are exposed on their side is steeper, while the more gentle slope on the other side is called a dip slope. cuestra
SOURCES & Links of Interest:
Blog on concretions
Thunder Egg page
GSA abstract on Calcite cementation in sandstone More on the Bashti
Mozley, P.S., 1996, The internal structure of carbonate concretions: A critical evaluation of the concentric model of concretion growth: Sedimentary Geology: v. 103, p. 85-91.