One of geology's more fascinating mysteries concerns the formation of concretions. Concretions are structures within rock that differ in form and/or composition from the matrix. Often, they form around an impurity of some sort, say, a tiny fossil. If concretions were all nicely spherical or crystalline in shape, we might be able to explain them as we do with the oyster's pearl and winter's snow-flake. Unfortunately for the theorists, concretions usually come in bizarre shapes -- shapes an avant garde sculptor might appreciate. Not only do concretions come in weird geometries but they may be replicated in prodigious numbers, like the famous Kimmeridge "coal money." Additionally, some flint concretions are arrayed in thick chalk beds in amazingly regular three-dimensional arrays that tax the ingenuity of any theorist.
To illustrate the extremes of nature's inorganic-chemical imagination, we now provide some illustrations from a recent two-part article in Rocks & Minerals and one of our catalog volumes.
(Dietrich, R.V.; "Carbonate Concretions,' Rocks & Minerals, 74:266 and 74:335, 1999. ESA3 in Neglected Geological Anomalies.)
Carbonate concretions ("imatra stones") from Finland. Virtually identical concretions occur in the Connecticut River Valley.
Vertical lines of flint concretions in chalk cliffs near Norfolk, England. Presumably the 3-dimensional array continues in the unexposed matrix behind the visible lines. (From: Neglected Geological Anomalies)
Mace-shaped sand concretions from Mt. Signal Tower, California. The longest measures 28 inches. (From: Neglected Geological Anomalies)