GMS Field Trip April 2025

GMS Field Trip
Agate, Jasper, Oolitic Jasper, and Fossils in Alabama
Saturday, April 12, 2025

What a beautiful day to play in gravel! Cool but not cold, warm but not hot, breezy but not windy – perfect! It rained a day or so before the trip too, so the gravel was washed and there were some convenient puddles for rinsing rocks.

Finds for the day included a coarse-grained blue rock that we have not identified yet (a first for this location), agates, concretions, jaspers, oolitic jaspers, and a few fossils.

One of the juniors found the first agate of the day. It was his first time in these gravel piles, but he acclimated quickly. His agate is particularly nice for this location. Most agates here are iron-stained, mostly brown, with fine banding that has very little contrast. This one has contrasting red, yellow, and gray banding within the brown. Most of the other agates were more like the usual brown agates, but there were also a few reminiscent of agate found in northwest Georgia. Compared to the brown agates, they may or may not be water-worn and rounded; they have agate banding randomly throughout the piece with areas that are not agate; the banding is larger with more color, especially pink, and has more contrast; some have brecciated areas; and stalagmitic areas reveal “bullseyes.” A large specimen found this time has a stalagmitic appearance with pink druzy quartz.

Juniors found agates and other interesting rocks all day, each time bringing them to me and other members for identification. They were curious about concretions and how they form. I explained how it all starts with a nucleus. It can be almost anything, including organic matter like a piece of shell. Dissolved minerals seep through sediment and precipitate onto the nucleus. More and more of the mineral precipitates, cementing more sediment around the nucleus. Concretions become harder than the surrounding sediment, so when the sediment weathers away, the concretion is left behind. One concretion had pyrite inside, something we have seen here only one time before. Concretions can form in interesting shapes that are fun to collect simply for the shape. If they have a hole through the middle, it has been said that they might be magical, so members were thrilled to find them.

Another type of concretion is an oolith (oh-uh-lith). They are super tiny, sand-sized concretions that form in shallow marine environments with mild turbulence and a high concentration of calcium carbonate in the water. Calcium carbonate precipitates from the water and builds up in layers around a nucleus to form an oolith. Ooliths can become cemented together to form oolite (oh-uh-light), basically a rock made of ooliths. Typically, oolite starts as a limestone, but over time, it can become silicified, i.e., the calcium carbonate is replaced by silica and forms oolitic jasper. Members found several nice specimens of oolitic jasper. I particularly like specimens that have whole ooliths, so you can see the spherical shape, plus ooliths that have broken open, so you can see the concentric layers inside.

There is a common misconception that ooliths are fossilized fish eggs, but as we know, they are not. There were some fossils in the gravel though. One member found an incredible trilobite cast in a concretion. The cast was very detailed, including the eyes! I found an unassuming rock with a sweet little gastropod fossil surprise, one of the best we have ever seen here. One of the juniors found a bryozoan fossil, easily identified by its mesh-like appearance. It can be difficult to distinguish petrified wood here from quartz with a woody look, but Charles found a piece that we think is truly petrified wood. Brooksellas, now considered pseudo-fossils (thanks to GMS member Dr. Morrison Nolan and his colleagues), have been found here before, but juniors once again astounded us with a superb brooksella!

Every time we are here, members find piles of pretty jaspers. Some jaspers are easy to see as a whole piece of gravel and are perfect for tumbling. Some jaspers can be seen only via a break in the gravel. And sometimes, a conchoidal fracture reveals a jasper window that is more art than specimen. One of the juniors found an exquisite example of this. Speaking of art, I suspect one very large and colorful jasper I saw is destined for some lapidary work.

We often see coarse-grained, loosely compacted iron sandstone here too. This time, there was a specimen with large botryoidal areas. Now, our curiosity is piqued and we want to learn more about this sandstone, especially how a botryoidal one could have formed.

Everyone had fun and went home with many pieces to add to their collections, tumble, or use in lapidary projects. This trip would not be possible without the generosity of the plant manager who always makes sure we are safe and that we have plenty of gravel to peruse. Many thanks to him and to the gravel company. Thank you to members for bringing their rocks to me so I could photograph them for this report, and for sharing their day with us. As always, thank you to Charles for arranging another great day in the gravel!

Note: Specific location information is not included in this report to respect the privacy of property owners.

Lori Carter on behalf of Charles Carter
e-mail: Field Trip email