The listing coordinates take you just outside of Mitchell Park, on the corner of 23rd Street NW and S Street NW.
In 1980, the park received a substantial renovation and on the corners of 23rd and Bancroft PI and 23rd and S Street the steep slopes of the park were reinforced with a pattern of large, irregular stones cemented together.
The used stone is sandstone that origins from the Oriskany Formation, in the northeastern of the United States. Taking a closer look at these stones reveals they are not quite so ordinary.
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How do fossils form?
Fossils are remnants and traces of plants and animals that have been preserved in rock. A fossil arises when an animal or plant does not completely decay, but is buried under a sediment. Because the sediment of the rock ensures the inclusion of plant or animal, there is no contact with oxygen and no further decomposition can take place. Over time, overlying deposits of sediments compressed the sediment where the fossil is buried, and calcium-rich liquid cemented the quartz grains together, around the fossil. The fossil itself does not have to be "petrified" but can be preserved in its original form. Another option is that eventually the original plant or animal does dissolve, leaving the detailed molds visible in the stones today.
Index fossils
A fossil is more than just a hardened relic from antiquity. The so-called index fossils are a special group of fossils, used by researchers to find out in which period a particular type of rock has been formed.
In order to function as a index fossil, a fossil must be clearly recognizable and originate from a plant or animal that has spread widely over as short a period as possible. When many of the same fossils are spread through several layers of a rock, it is easy to determine when a type of stone has been formed. The accuracy for determining the dating of a rock layer also depends on how long a particular species has lived.
Examples of Brachiopod fossils can be found in the graph, which assigns three types of Brachiopod fossils of different geological eras. The gray bar indicates true which of the eras the different Brachiopods have lived. This is usefull for determing true which eras a type of stone has been formed. In this example only two out of three Brachiopod fossils are useful as index fossil. The Lingula Brachiopod already existed in the Cambrian era, and still does today. |
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Brachiopod fossils
At first these marine creatures look like ordinary sea shells. But this is a deception, these animals are more closely related to primitive worms than with shells. The size of brachiopods is from a few millimeters up to 8cm. They were fastened with a stalk to the seabed and filtered their food out of the passing water. Even if they did not form colonies, they lived very often in groups close together on the seafloor.
1. Lingula A shell encloses the organs and other internal parts of the body, except for a long, fleshy stalk that extends from the tail end of the shell. The shell has two nearly identical tan or bronze valves, composed of chitin, protein, and calcium phosphate covered by a thin glossy layer of protein called periostracum, which gives it a smooth surface.
2. Spiriferida These Brachiopod fossils ar an order of extinct articulate which are known for their long hinge line, which is often the widest part of the shell. In some genera of the Spiriferida the hinge is greatly elongated, giving them a wing like appearance. They also often have a deep fold down the center of the shell. Spiriferida first appear in the Late Ordovician era, survived the great Permian extinction, finally becoming extinct during the Early to Middle Jurassic.
3. Obolellata The Obolellata are a class of Rhynchonelliform brachiopods with two orders, Obolellida and Naukatida. The Obolellata shell is typically impunctate, biconvex, and oval or subcircular in shape. They are like other inarticulates in that the shell has no hinge. They are essentially restricted to the Cambrian era.
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Go to the listing coordinates and look closely at the stones used on the steep slope. You don't have to climb up the slope, everything you need to see can be seen from the sidewalk.
1. Which of the Brachiopod fossils mentioned in the listing is most common here?
2. Now that you have found the most common Brachiopod here, try using the graph in the listing to determine between which periods and eras the stones originated.
3. Optional Pictures of your geological quest are always welcome.
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