Fossil of the Week

8/11/10 - Concretion

Although this might look like a fossilized turtle shell, it is neither a turtle nor a shell. It's a concretion, and a big one, approximately 30 centimeters (12 inches) in diameter. Concretions are not fossils at all. They are common geologic phenomena found in all types of sedimentary rock, especially the shales and siltstones of central New York, so they are quite common in the Ithaca area. Concretions form when minerals (such as calcium carbonate, which forms limestone) within soft sediment precipitate in concentric layers around a nucleus, such as a shell or pebble. So a concretion often actually contains a fossil. Concretions are spherical to oval masses, and are harder than the surrounding rock, so the rock weathers first, exposing the concretion. They can range in size from less than an inch to several feet in diameter. Sometimes the concretion will crack and the cracks will fill with matrix, forming a rounded rock that looks like a turtle shell, like this one here.

Descriptions of concretions date from geological publications of the 18th century. They have been misinterpreted as dinosaur eggs, animal and plant fossils, extraterrestrial debris, and human artifacts. Bowling Balls Beach in Mendocino County, California, is well known for its ball-shaped concretions that weather out of the Cenozoic mudstone. Theodore Roosevelt National Park in North Dakota has concretions that are almost 10 feet in diameter. Small hematite concretions, called "blueberries," have even been observed on the planet Mars. Other names that have been applied to concretions are Kansas Pop Rocks, Moki Marbles, Koutu Boulders, septaria, cannonballs, and doggers.

Text by Paula Mikkelsen