Where is Iron Concretion found?
Iron oxide 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
Famous for 'Moqui Marbles' (Navajo for 'dead people's brains'), which are small, spherical iron concretions found in the Navajo Sandstone. These are typically hematite-rich and often have a harder outer shell and a softer, sandy interior.
North Dakota, USA
Large 'cannonball' concretions are found in the Cannonball River area, weathering out of the Paleocene Cannonball Formation.
New Zealand
Large, spherical concretions (Koutu Boulders, Moeraki Boulders) are found along coastlines, weathering out of mudstones and siltstones. While many are calcite-cemented, some are iron-rich.
United Kingdom
Ironstone concretions are common in Jurassic and Cretaceous sedimentary sequences, particularly in areas like Yorkshire and the Isle of Wight.
Globally
Iron concretions are a common diagenetic feature in sedimentary basins worldwide, wherever iron-rich groundwater has interacted with permeable sediments.
Collecting tips
- Look for resistant features. Concretions are typically harder than the surrounding host rock, so they often weather out and accumulate on the surface, especially in arid or semi-arid environments, or along riverbeds and coastlines where erosion is active.
- Examine sedimentary outcrops. Look for spherical or ovoid structures embedded within layers of sandstone, siltstone, or shale. They may appear as distinct, darker-colored masses.
- Check for 'cannonball' shapes. Many iron concretions are remarkably spherical, resembling cannonballs. This shape is a strong indicator.
- Test hardness. Iron concretions are generally quite hard. A steel knife or file will not easily scratch them, distinguishing them from softer mudballs or clay clasts.
- Observe color and streak. The characteristic reddish-brown to black color and a reddish-brown streak (for hematite-rich types) or yellowish-brown streak (for goethite-rich types) are good indicators.
Quick Check
- Color:
- Reddish-brown, dark brown, black, yellowish-brown
- Luster:
- Dull to earthy, sometimes sub-metallic
- Streak:
- Reddish-brown (hematite) or yellowish-brown (goethite/limonite)
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