The Arnheim Formation, named after the village of Arnheim in Brown County, Ohio, is a crucial component of the region’s geological history. Dating back to the Upper Ordovician period—approximately 445 million years ago—this formation consists of interbedded fossiliferous limestones and shales that reveal a dynamic ancient marine environment. Notably, it includes the "Mount Orab butter shale" bed, a soft claystone layer renowned for its exceptional fossil preservation. The Arnheim Formation provides a vital record of prehistoric marine life, preserving trilobites like Flexicalymene retrorsa and the giant Isotelus maximus, alongside an array of brachiopods, bryozoans, and crinoids. Its study continues to shed light on the evolution and biodiversity of ancient seafloor ecosystems.

Flat Run Fossils is a BYOT (Bring Your Own Tools) site, meaning visitors must come prepared with the right equipment to maximize their fossil-hunting experience. The terrain consists of exposed sedimentary layers, including fossiliferous limestones and shales, which require proper tools for extracting specimens safely and effectively.

🔴 Required Safety Gear

• Eye Protection (Mandatory): Safety glasses or goggles are required at all times.
• Gloves: Protective gloves help prevent cuts and scrapes while handling rocks and tools.
• Sturdy Footwear: Closed-toe shoes or boots with good traction are required to navigate uneven terrain.

Recommended Tools

• Geological Hammer: Essential for splitting shale and extracting fossils with precision.
• Medium to Large Chisels: Used for breaking apart rock layers to reveal fossils.
• Pry Bars: Helps loosen larger rock slabs and access deeper fossil-bearing layers.
• Storage & Transport: Bring a sturdy bucket, padded containers, or wrapping materials (such as newspaper or foam) to safely store and transport your fossil finds.
• Brushes & Picks: Small brushes and dental picks help with delicate fossil cleaning on-site.
• Padding Knee pads or foam pad for kneeling on. Its a game changer

Additional Tips

Flat Run Fossils is known for its well-preserved trilobites and other marine fossils from the Upper Ordovician. To improve your chances of success, focus on splitting shale carefully and inspecting exposed layers. Be mindful of the terrain and ensure you leave the site as you found it—ethical fossil collecting helps preserve the experience for future visitors.

Flat Run Fossils is home to an exceptional fossil record of two iconic trilobite species: Flexicalymene retrorsa and Isotelus maximus. These ancient arthropods thrived in the warm, shallow seas of the Upper Ordovician period, around 445 million years ago, when Ohio was submerged beneath a vast inland sea. The fossils found here provide extraordinary insight into prehistoric marine ecosystems, preserving details that help scientists and collectors understand their behaviors and environments.

Flexicalymene retrorsa, a smaller trilobite commonly found in the region’s soft shale layers, is especially well-known for its distinctive enrolled posture. This defensive behavior, where the trilobite curled into a tight ball to protect itself from predators, is frequently preserved in fossils found at the Mount Orab butter shale and other layers within the Arnheim Formation. Many of these fossils remain completely intact, with delicate details of the exoskeleton clearly visible, suggesting rapid burial in fine mud before scavengers or decay could destroy them.

Isotelus maximus, the largest trilobite species ever discovered, is a rare but highly prized find at Flat Run. Some specimens reach lengths of up to 15 inches (38 cm), making them among the largest trilobites in the world. Their smooth, oval-shaped bodies suggest a life spent partially buried in soft sediment, gliding across the seafloor in search of food. This species' immense size and significance have earned it the designation of Ohio’s official state fossil, highlighting its importance in both science and local heritage.

Fossils from the Arnheim Formation—including these remarkable trilobites—tell the story of an ancient ocean teeming with life. Each discovery offers a snapshot of a long-lost ecosystem, preserving the evolutionary history of organisms that once dominated Earth's seas. Whether you're a fossil collector, paleontology enthusiast, or curious explorer, Flat Run Fossils provides a unique opportunity to connect with Ohio’s deep prehistoric past.