35 million years in the making
Along a creek or river bed… this would look like a rather ordinary large pebble.
But, that assumption is far from the truth… this particular geologic formation is known as a concretion.
“A concretion is a compact mass of mineral matter, usually spherical or disk-shaped, embedded in a host rock of a different composition. This hard, round mass of sedimentary rock cement is carried into place by ground water.
They usually form early in the burial history of sediment, before the rest of the sediment has hardened into solid rock. Concretions, the most varied-shaped rocks of the sedimentary world, occur when a considerable amount of cementing material collects locally around a nucleus, often organic, such as a leaf, tooth, piece of shell or fossil, dead and/or decade matter – like a crab or fish.
Most concretions form around marine invertebrates…but are not limited to marine life.
Concretions vary in size, shape, hardness, and color, from objects that require a magnifying lens to be clearly visible to huge bodies 10 feet in diameter and weighing several hundred pounds.”
From-What on Earth is a Concretion? by Steven Michael
Inside this concretion a fossil gem!
A fossil crab… Pulalius vulgaris
- Class Malacostraca, Order Decapoda, Infraorder Brachyura, Superfamily Xanthoidea
- Geological Time: Eocene; 35 millions years ago
- Fossil Site: Lincoln Creek Formation, Porter, Washington
Among the world’s best-preserved crab fossils are those occurring in western Washington state. Generally the crabs quickly fall apart after death. So for these crabs to be found is so fine a state of preservation in a rarity. They must have been buried soon after death. The crabs are found in concretions along the sides of creeks and beds of local streams. Painstaking work and long hours are required to prepare them. (Information from: Fossil Mall.com)