The word "concretion" is derived from the Latin words "con" – meaning "together" – and "cresco" – meaning "to grow."
A rock that grows? Is there really such a thing?
...so what exactly is 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. Concretions are world wide and very common, but here in Oregon, the largest concentration of concretions are found throughout the Cape Arago Headland. These locations includes Yoakum Point, Sunset Bay, Norton Gulch, Shore Acres, the Simpson Reef Mainland, and the "giants", found near Fossil Point.
These "grown together" rocks have a variety of origins that require geologists to integrate information from a variety of disciplines, including biology, chemistry, soil science, meteorology and geology itself. In this photo, my mother is standing above a concretion...or I should say, half a concretion. Half of the rock has fallen away, while the remaining half has not eroded out. This "side cut" profile looks like what half of a jaw-breaker would resemble.Concretions are commonly misunderstood geologic structures. Descriptions dating from the 18th century attest to the fact that concretions have long been regarded as geological curiosities. Concretions are often mistaken for dinosaur fossil eggs, turtle eggs, or shells, or bones, human artifacts and even extraterrestrial debris. They are actually not fossils, artifacts, or even debris from space, but a very common geologic phenomenon in all types of sedimentary rock; which include sandstone, shale, siltstone, and limestone. In this photo, my buddy Gus is sitting amongst a collection of concretions partially embedded in a sandstone cliff on the south wall of Simpson Beach at Shore Acres State Park. Slowly eroding out, at Shore Acres State Park, this concretion has recently revealed itself. Eventually it will erode completely out of the cliff...but when, only time will tell.Much like taking kids into a candy store, here, my friends Buddy and Nicole, collect concretions with much excitement from a rocky beach unofficially named "Concretion Cove". Their great finds are used as artwork in their living room. My Friends Brian and Tyler-Marie collected their share of treasures as well. You can find concretions from the size of golf balls to the size of bowling balls in "Concretion Cove" Gus stands next to one of my favorite concretions. In this photo, the size of the rock is apparently no larger than a softball...but scroll back up to the first photo. (It is the same rock, artistically captured to appear much larger than it really is.)
In this photo, Gus is standing in front of unusually shaped concretions along the Yoakum Point Concretion Wall complex.Some of the best concretions to photograph are in places that are difficult to get too. Some cliff-hanging rock climbing is required...but not recommended.
Concretions throughout the Cape Arago Headland are comprised mostly of silica, embedded in sandstone. Silica gives them a strength so solid that attempting to break them open only results in a destroyed rock of rubble...that is if you can break them at all.Concretions exposed on the southern cliffs of Shore Acres State Park.
The "Giant" Concretions are located just north of Fossil Point - along the Coos River near the Charleston area. This phenomenal collection of concretions consist of anywhere between 150 to 170 rocks. These giant concretions do not indicate that the nucleus that started the formation is of a large format. These rocks could have also formed around the same size leaf, tooth or crab as those of the baseballs and bowling balls. Their size though could indicate the conditions were perfect to form over a longer period of time...allowing them to gain "giant" status. In this photo, a giant concretion is slowly eroding out away from the softer rock which entombed it. Only the rock's top 1/4 is currently exposed. Once out, the rock will be one of the largest "giants" in the area. Just how long it will take to be completely freed will depend on mother nature. To visit the "giants" it is recommended to go at low tide. Though you can still get to them at a higher tide, most will be underwater. It is also recommended to wear a good pair of boots...the area is very gooey and slippery.Brian, Tyler-Marie, and I took one full day and visited the five concretion complexes within the Cape Arago Headland...from the smallest marbles at Sunset Bay, to the "giants" of Fossil Point. In this photo, I would love to pry open this giant concretion and see what started the process. All I'd have to do is lift the top half off...I never get tired of visiting, exploring, investigating and collecting these unusual natural formations. Lakota and Tyler-Marie happily pose next to the largest "giant" concretion within the Fossil Point complex. Though the Cape Arago Headland has the largest concentration of concretions, there are other Oregon locations that inhabit these phenomenal formations. From the Cape Arago Headland areas, they form in pockets in a south-southeast direction. Concretions reveal themselves along landslides, highway road cuts, logging roads, streams, rivers, and rock quarries.Concretions can even be found in masses at the 2000 foot level, shown here at the Upper Coquille River Falls. Concretions can also be found throughout Cape Blanco, and the Loon Lake area, east of Reedsport.
Oregon law prohibits removing concretions that are embedded in a host rock. It is illegal and can be punishable by law. If the concretions are not attached to its host rock, then collecting is permitted, as long as they are collected by hand, and no tool or machinery is used to collect them and get them back to your vehicle.
The following pictures are of some of my favorite concretions within my personal collection.
This shape of concretion is unofficially known as a "Tear-drop -or- Raindrop" The rock is approximately seven inches wide and about twelve inches tall, and VERY HEAVY! One of my favorites!
A friend of mine recently called this duel concretion a "Faternity Doll". It is more commonly refered to as a "Peanut". Two concretions growing close together have merged into one.
The following concretion is a great example of showing how concretions are not all round. The two inch round concretion in the center grew into a tree branch also forming into a concretion. I found this specimen within a landside near the Cape Arago Lighthouse. The branch concretion was broken in several spots. I collected as many pieces of the branch and glued them back together. It is such a great specimen. Once a concretion is "successfully" broken open, it can reveal a hidden treasure. I found the following three inch concretion at Sunset Bay. It was a solid round rock at the time. I brought it home and placed it in the yard. Eventually, time and weather cracked the rock into four pieces, revealing the nucleus that started the concretion process...a mussel shell. The concretions throughout the Cape Arago Headland will mostly consist of a shell nucleus. If you're wanting to find a rare treasure of the nucleus being a crab, you'll want to venture into the mountains above Powers, Oregon. At around 2000 feet above sea level, oval shaped concretions, likely to house crabs, abound. The rock consistancy is softer and easier to break open.
This excellent example of a crab concretion was recently sold on Ebay for over $400 dollars. The owner stated it was collected in the mountains near Powers. (I did not buy it!)