A Gallery of Concretions

Ferruginous Gravel, Australia

Concretions are hard bodies that form in sediments before they become sedimentary rocks. Slow chemical changes, perhaps related to microbial activity, cause minerals to come out of the groundwater and seal the sediment together. Most often the cementing mineral is calcite, but the brown, iron-bearing carbonate mineral siderite is also common. Some concretions have a central particle, such as a fossil, that triggered the cementation. Others have a void, perhaps where a central object dissolved away, and others have nothing special inside, maybe because the cementation was imposed from outside.

A concretion consists of the same material as the rock around it, plus the cementing mineral, whereas a nodule (like flint nodules in limestone) is composed of different material.

Concretions can be shaped like cylinders, sheets, nearly perfect spheres, and everything in between. Most are spherical. In size, they can range from as small as gravel to as large as a truck. This gallery shows concretions that range in size from small to large.

These gravel-size concretions of iron-bearing (ferruginous) material are from Sugarloaf Reservoir Park, Victoria, Australia.

Root-Cast Concretion, California

This small cylindrical concretion formed around the trace of a plant root in shale of Miocene age from Sonoma County, California.

Concretions from Louisiana

Concretions from Cenozoic rocks of the Claiborne Group of Louisiana and Arkansas. The iron cement includes the amorphous oxide mixture limonite. 

Mushroom Shaped Concretion, Topeka, Kansas

This concretion appears to owe its mushroom shape from a short period of erosion after it broke in half, exposing its core. Concretions may be quite fragile.

Conglomeratic Concretion

Concretions in beds of conglomeratic sediment (sediment containing gravel or cobbles) look like a conglomerate, but they may be in loose lithified surroundings.

Concretion from South Africa

Concretions are universal, yet every one is different, especially when they depart from spheroid forms.

Bone-Shaped Concretion

Concretions often assume organic shapes, which catch people's eyes. Early geological thinkers had to learn to differentiate them from genuine fossils.

Tubular Concretions, Wyoming

This concretion in Flaming Gorge may have arisen from a root, a burrow or a bone -- or something else.

Ironstone Concretion, Iowa

The curvilinear shapes of concretions are suggestive of organic remains or fossils. This photo was posted in the Geology Forum.

Concretion, Genessee Shale, New York

Concretion from the Genesee Shale, of Devonian age, in the Letchworth State Park museum, New York. This appears to have grown as a soft mineral gel.

Concretion in Claystone, California

Interior of a pod-shaped ferruginous concretion that formed in shale of Eocene age in Oakland, California.

Concretions in Shale, New York

Concretions from the Marcellus Shale near Bethany, New York. The bumps on the right-hand one are fossil shells; planes on the left-hand one are fissure fillings.

Concretion Cross Section, Iran

This concretion from the Gorgan region of Iran displays its inner layers in cross section. The upper flat surface may be a bedding plane of the shale host rock.

Pennsylvania Concretion

Many people are convinced that their concretion is a dinosaur egg or similar fossil, but no egg in the world has ever been as large as this specimen.

Ironstone Concretions, England

Large, irregular concretions in the Scalby Formation (Middle Jurassic age) at Burniston Bay near Scarborough, U.K. ​The knife handle is 8 centimeters long.

Concretion with Crossbedding, Montana

These Montana concretions eroded from the sand beds behind them. Crossbedding from the sand is now preserved in the rocks.

Concretion Hoodoo, Montana

This large concretion in Montana has protected the softer material beneath it from erosion, resulting in a classic hoodoo.

Concretions, Scotland

Large ironstone (ferruginous) concretions in Jurassic rocks of Laig Bay in Isle of Eigg, Scotland.

Bowling Ball Beach, California

This locality is near Point Arena, part of Schooner Gulch State Beach. Concretions weather out of steeply tilted mudstone of Cenozoic age.

Concretions at Bowling Ball Beach

Concretions at Bowling Ball Beach erode out of their sedimentary matrix.

Moeraki Boulder Concretions

Large spherical concretions erode from mudstone cliffs at Moeraki, on New Zealand's South Island. These grew soon after the sediment was deposited.

Eroded Concretions at Moeraki, New Zealand

The outer part of the Moeraki boulders erodes to reveal the inner septarian veins of calcite, which grew outward from a hollow core.

Broken Concretion at Moeraki

This large fragment reveals the inner structure of the septarian concretions at Moeraki, New Zealand. This site is a scientific reserve.

Giant Concretions in Alberta, Canada

The Grand Rapids in remote northern Alberta may have the world's largest concretions. They create white water rapids in the Athabasca River.