July 31, 2003
Concretions are hard masses of sedimentary and, more rarely, volcanic rock that form by the preferential precipitation of minerals (cementation) in localized portions of the rock. They're commonly sub-spherical, but frequently form a variety of other shapes, including disks, grape-like aggregates, and complex shapes that defy description. Concretions are usually very noticeable features, because they have a strikingly different color and/or hardness than the rest of the rock. In some areas this is unfortunate, as the concretions have attracted the unwanted attention of local graffiti artists. Commonly, when you break open concretions you'll find that they have formed around a nucleus, such as a fossil fragment or piece of organic matter. For a variety of reasons, this nucleus created a more favorable site for cement precipitation than other sites in the rock.
The 2 m by 3 m sandstone concretion on the left of the above photo (taken in October 2002) was sliced nicely in half by the excavation of the I-70 roadcut through the Dakota Hogback, west of Denver, Colorado. This particular outcrop is easily reached by a short hike along the "geology trail" on the south side of the highway. Another smaller but whole concretion is exposed south of here where Alameda Drive crosses Dinosaur Ridge. In this area, the rocks of the Dakota Formation also preserve numerous dinosaur track-ways in the rippled and petrified sands of an early Cretaceous shoreline.(see Earth Science Picture of the Day for May 12, 2003). The photo on the right was taken in June, 1984. It shows Bill Cordua standing by a large septarian concretion, one of the Moeraki Boulders. These boulders are found on the beach along the east coast of South Island, New Zealand (between the towns of Moeraki and Hampton). This area is a scientific preserve. The concretions are weathering out of Paleocene mudstones and may reach 2 m in diameter. It's estimated that the largest of these concretions took about 4 million years to grow.