Where is Iron Concretion found?
Iron oxide/hydroxide concretion
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Open the appIron Concretion 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 Iron Concretion
Utah, USA
The Navajo Sandstone in southern Utah is famous for producing 'Moqui Marbles' or 'Moqui Balls,' which are iron concretions composed primarily of hematite and goethite, often with a sandstone core. These are particularly abundant in areas like Zion National Park and Grand Staircase-Escalante National Monument.
Kansas, USA
Large, spherical iron concretions are found in the Cretaceous Dakota Sandstone in north-central Kansas, often weathering out to form prominent features in stream beds and outcrops.
North Dakota, USA
The Cannonball River area and other parts of North Dakota contain numerous iron concretions, often spherical or disc-shaped, weathering out of Tertiary sedimentary formations.
England, UK
Ironstone concretions are common in Jurassic and Cretaceous sedimentary sequences, particularly in the Cleveland Ironstone Formation and the Wealden Group, where they were historically mined for iron ore.
Australia
Extensive lateritic iron concretions (pisoliths and ooliths) are found in weathered profiles across Western Australia and other arid regions, forming significant iron ore deposits.
Worldwide
Iron concretions are globally ubiquitous and can be found in almost any sedimentary basin or soil profile where the conditions for their formation (iron-rich fluids, redox gradients, porous host material) are met.
Collecting tips
- Look in Sedimentary Rocks. Focus your search in areas with exposed sedimentary rock layers, particularly sandstones, shales, and siltstones. Concretions are often harder than the surrounding rock and will weather out, accumulating at the base of outcrops or in stream beds.
- Check for Weathering. Concretions are more resistant to erosion than their host rock. Look for rounded or oddly shaped rocks that stand out from the surrounding sediment, especially in areas where erosion has exposed older layers.
- Examine Stream Beds and Washes. Water erosion often concentrates concretions in stream beds, dry washes, and alluvial fans, as they are transported less easily than finer sediment.
- Investigate Soil Profiles. In some regions, iron concretions (e.g., lateritic concretions, bog iron) can be found within soil horizons, particularly in areas with fluctuating water tables.
- Observe Color and Density. Iron concretions will typically be darker (red, brown, black) and noticeably denser than the surrounding host rock. Their weight can be a good indicator.
Quick Check
- Color:
- Reddish-brown, yellowish-brown, dark brown, black
- Luster:
- Dull to earthy, sometimes sub-metallic
- Streak:
- Reddish-brown (hematite-rich) or yellowish-brown (goethite-rich)
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