CONCRETIONS The treasure chests of geology at Highbanks Beth Renner

hile wading through one of the stools conveniently placed along the creek- They’re essentially balls of geologic his- Wstreams at Highbanks, I looked up at side like sculptures. I couldn’t believe that I tory, with their core contents a mystery and my surroundings and began to notice what had never seen this before and better yet it they are right here in Ohio. was all around me. I had been staring at is all-natural. To uncover the mystery, one must imag- my feet as I waded through the ine the area during the Devonian Period water, making every attempt some 350 million years ago. Back then not to slip and get soaked. Ohio was covered by a muddy, shallow It was only when I stopped ocean. caring about slipping into the As time went on, mud from the creek that I started learning. A waters would settle on the ocean large lump protruding from the floor forming Ohio’s sedimen- water first sparked my interest. tary bedrock of limestone and “Is that cannonball thing a concretion?” shale. I remember asking myself. Concretions developed at the same As I walked around a slight bend in the time as the shale, creating geologic stream, more concretions appeared treasure chests. Fish or plants living and I became quite curious. They in the ancient ocean would die and seemed to be everywhere. sink to the bottom of the sea. As they I saw some partially submerged in decayed, they became the center of the the creek bed, others along the banks of the Jaw bone of Dinichthys concretion, or nucleus. water and even some embedded in the rock I wondered how I would be able to In many cases, the nucleus consists of walls that created the ravine around me. describe these ancient masses of mineral an ancient life form which can be in the A number of the concretions were so matter and let others know how cool form of a fossil, like that of a fish or some large that they had begun to crack and they are to come upon when creeking at form of prehistoric vegetation. Over many erode and now resembled low-lying round Highbanks. hundreds of thousands of years, the par-

8 Parkscope ticles and sediment bond together and the concretion develops.

Through slow chemical changes, Cheryl Blair minerals such as iron are extracted from the forming mass, causing its remnants to cement together, building the concretion. Often, they are harder than the material in which they came from. The process of erosion is responsible for revealing the concretions in the sedimen- tary rock. The power of water and wind, over time, has gradually deteriorated the soft shale rock leaving the much more solid concretions buried within it exposed. This is especially true in the case of Highbanks’ concretions, as shale is easily eroded and breakable. Ohio Shale is visibly distinguished from A naturalist-led hike through the Highbanks ravines is the best way to see concretions. other rocks by its appearance. It is typically Otherwise, the nucleus has dissolved over dark gray in color and is easily breakable, time leaving a void in the center of the frequently weathering into small brown concretion. colored chips or flakes. Beth Renner Concretions are often orange and brown Shale is a fine-grained sedimentary in color due to the high iron content and rock. This means that it forms from the can vary widely in size and in shape. compaction and gradual collection of more They may be very small and manage- sediment and particles layer upon layer. It able, like a bowling ball, or they could be is a laminated rock, which simply means it the size of a truck. As far as shape goes, is composed of many thin layers. concretions are typically round or tubular The ravine I was wading through was in shape, however some may be flat, or, my exactly like this. The horizontal shale layers favorite, near perfect spheres. were abundantly clear; so much so that A remarkable discovery took place at they resembled towering stacks of disorga- Highbanks in the late 1980s. The jawbone nized, layered paper. of a prehistoric fish called a Dinichthys As I continued wading through the creek was uncovered in the center of an eroding I touched the thick shale walls and pieces concretion that had split in half. of shale crumbled under the pressure of my The next time you’re at the park stop by fingertips. It was neat to witness rocks so the nature center and check the Dinichthys delicate when exposed out in the open. display. The ancient fish could reach 18 feet I couldn’t help but admire the beauti- Water erosion can cause a depression in length and weigh up to four tons. In the ful shale that composed the creek bed and or void in the center of a concretion. Nature Center, you will see a replica jaw lined the walls of the ravine. As I walked on bone of the armor-plated ancient predatory Be sure to leave them where they were the dryer portions of the creek bed, consist- fish, as well as its equally ferocious cousin and avoid damaging them or splitting ing almost exclusively of crumbled shale, the Dunkleosteus. them open. It is important to leave them I could hear it crunching and cracking Concretions are interesting and mys- so future park visitors can experience their beneath my feet, like walking on potato terious and winter is a great time to hike beauty and mystery for years to come. n chips. Highbanks’ Dripping Rock Trail and look After experiencing a beautiful creek for them in the ravine below. If concretions featuring concretions, I decided to learn are ever discovered in your explorations Beth Renner more about them. The composition of Ohio please treat them with care. Highbanks Nature Center Attendant concretions typically includes other miner- als like calcite, dolomite, silica and pyrite. In some cases, the center or nucleus Off-Trail Ravine Hike: Sun Jan 12, 2014 – 1pm of the concretion is still present within it Explore the Highbanks ravines and see the concretions on a 3-mile off-trail hike. and it may even contain an intact fossil.

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