Where is Iron-rich concretion found?
Goethite/Limonite concretion
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Open the appIron-rich 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-rich concretion
Midwestern United States
Abundant in Cretaceous sandstones (e.g., Dakota Formation) and glacial tills, often weathering out as 'ironstone concretions' or 'Indian paint pots'.
Utah, USA
Famous for 'Moqui Marbles' (Navajo Sandstone concretions), which are iron-rich concretions with a hematite-rich outer shell and sandstone core.
England
Common in Jurassic and Cretaceous sedimentary rocks, such as the Dogger Formation and various clay formations.
Australia
Widespread in lateritic soils and weathered profiles, forming 'ironstone gravels' or 'ferricretes'.
Worldwide
Found globally in diverse sedimentary basins and soil profiles where conditions for iron precipitation exist.
Collecting tips
- Look in weathered sedimentary outcrops. Concretions are often more resistant to weathering than the surrounding host rock, causing them to stand out or erode free.
- Check stream beds and gravel pits. Eroded concretions can accumulate in these areas, especially if the source rock is nearby.
- Examine soil profiles. In some regions, iron-rich concretions (e.g., bog iron, lateritic nodules) form directly within soils.
- Note the host rock type. Concretions are common in sandstones, shales, and limestones. Understanding the local geology can guide your search.
Quick Check
- Color:
- Brown, yellow, reddish-brown
- Luster:
- Dull, earthy, sub-metallic
- Streak:
- Yellowish-brown to brownish-yellow (for goethite/limonite)
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