Where is Concretion with fossil core found?
Sedimentary concretion with fossil
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Open the appConcretion with fossil core occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Concretion with fossil core
Mazon Creek, Illinois, USA
Famous for exceptionally well-preserved plant and animal fossils (Pennsylvanian age) within ironstone concretions. These concretions are often found in shales and mudstones.
Kimmeridge Clay Formation, England
Known for concretions containing marine reptile fossils (Jurassic age), particularly ichthyosaurs and plesiosaurs.
Pierre Shale, Western Interior Seaway, USA
Yields numerous concretions with ammonites, marine reptiles, and other Cretaceous marine fossils.
Morrison Formation, Western USA
While less common for concretions with large vertebrate fossils, smaller concretions with plant or invertebrate remains can be found in some horizons (Jurassic age).
Various coastal and riverine exposures worldwide
Concretions with fossil cores can be found in eroded sedimentary rock outcrops, riverbeds, and coastal cliffs where fossiliferous sedimentary layers are exposed.
Collecting tips
- Look for weathered-out nodules. Concretions are typically harder than the surrounding host rock and often weather out, accumulating at the base of outcrops, in stream beds, or on eroded surfaces. Look for rounded or ovoid stones that seem out of place.
- Examine sedimentary layers. Search in shales, mudstones, siltstones, and fine-grained sandstones. Concretions often form in specific horizons within these layers. Look for subtle bulges or harder masses within the rock.
- Check for concentric structures. Some concretions may show faint concentric layering on their exterior, indicating their growth pattern. This can help distinguish them from ordinary pebbles.
- Break open suspicious specimens (with care). If a concretion is found, carefully breaking it open (e.g., with a rock hammer and chisel, or by freezing/thawing if appropriate for the material) may reveal the fossil core. Always wear appropriate safety gear (eye protection, gloves).
- Research local geology. Familiarize yourself with the geology of your collecting area. Knowing which formations are known for concretions or fossils will significantly increase your chances of success.
Quick Check
- Color:
- Gray, brown, tan, yellow, reddish-brown (variable)
- Luster:
- Dull to earthy
- Streak:
- White to light brown (for carbonate concretions); variable for others
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