Logging Tasks
1. Sedimentary rocks made be clastic (made from smaller pieces of rock that are transported from somewhere else) or chemical (precipitated from minerals in the water, usually at or near the place where they are formed. Are these two gravestones clastic sedimentary rocks or chemical sedimentary rocks?
2. One gravestone was formed from living things (organic) and the other from non-living things (inorganic). What was the Grahame gravestone made from and was it living or non-living? What was the Wilton gravestone made from and was it living or non-living? If living, is it a plant or an animal?
3. What is the primary chemcial component of limestone?
Photo - include a photo, pointing to either a crinoid or an oolite. You may use a finger, pen/pencil, twig or any other creative option. Please DO NOT include which gravestone it is but DO say what you are pointing at.
NOTE: While it would be helpful to have a hand lens (10x is suitable) zooming in with the camera function of a smartphone works remarkably well. No special tools are required for this EarthCache.
Holywell Cemetery
Holywell Cemetery was created in 1847. Burials continued until the 1990's. There are over 1200 graves including some of Oxford's most influential figures. Luckliy you will only have to visit two of the gravesites for this EarthCache. But you are certainly encouraged to allow yourself time to explore the rest of this fascinating cemetery.
The coordinates will take you to the Holywell Cemetery entrance on St. Cross Road. From the sidewalk look for the signed, somewhat obscure path to the cemetery. From here it is only about 25 meters to your first gravestone (Grahame). The other is only about 30 meters beyond that (Wilton).
Types of Rock
The three main types of rock are igneous, sedimentary and metamorphic.
Sedimentary rock may be divided into two main categories, depending on how they were formed: clastic and chemical.
How Sedimentray Rocks Are Formed
Clastic Sedimentary Rocks
- Often sedimentary rocks are formed from weathered rock, breaking into smaller pieces, or clasts, and being transported and deposited, allowing sediment to accumulate.
- As subsequent layers accumulate on top, the sediment layers are compacted, squeezing the grains together and driving out the water.
- Cementing minerals coat the sediment grains, gluing them together into a fused rock.
In short, deposition, compaction and cementation turn loose sediment grains into clastic sedimentary rock. The entire process from deposition to rock formation is called lithification.
Chemical Sedimentary Rocks
Chemical sedimentary rocks are precipitated from water saturated with dissolved minerals.
Chemical sedimentary rocks may be of organic (derived from living things,) or inorganic (derived from non-living things) origin.
Limestone
Limestone is, by far, the most common chemical sedimentary rock.
Unlike clastic sedimentary rock, limestone is produced almost entirely from sediments originating at or near the place of deposition.
Here we are going to look at two gravestones, both made of a chemical sedimentary rock - limestone. One will be of organic origin (crinoids) and the other, inorganic (ooliths). As in all limestones, calcium carbonate (CaCO3) is the primary component in both the oolitic and crinoidal (fossiliferous) limestones.
The calcium carbonate particles that make up limestones come from many sources: skeletons and hard tissues of animals such as mollusks (bivalves and gastropods), echinoderms, corals, foraminifera, algae and a range of unicellular micro-organisms. When conditions like water temperature, salinity, and evaporation rates change, the dissolved calcium carbonate starts to precipitate out of the water.
Gravestone #1: Kenneth Grahame
The end of the Jurassic Period, about 145 million years ago, saw sea levels fall in southern England, and remain so for about 5 million years. Rocks of this age are found at the surface in Oxfordshire. The warm, shallow marine waters, were ideal conditions for the formation of oolites.
The precipitation of calcium carbonate typically begins around small particles, such as sand grains, shell fragments, or other debris that act as nuclei. As water moves over these particles, calcium carbonate is deposited in concentric layers, gradually building up around the particle. The layers of calcium carbonate become thicker as the particle continues to roll around in the water, often driven by wave action or currents.These concentric layers of calcium carbonate give the grains their spherical shape, forming the characteristic ooliths. Over time, large numbers of these small, spherical grains accumulate on the seafloor. When enough ooliths build up and become compacted, they can become cemented together by more calcium carbonate that precipitates out of the water, forming oolitic limestone.The calcium carbonate that precipitates around the ooliths not only creates the concentric layers but also acts as a natural cement. This binds the ooliths together into a solid rock.The resulting limestone has a characteristic appearance of tiny, round grains (ooliths) that are often visible to the naked eye.
(zoomed in view of oolites on surface of a gravestone)
Gravestone #2 Robert James Wilton
During the Lower Carboniferous Period, around 359 – 327 million years ago, a shallow sea covered most of southern England. Muddy, shelly limestones of this period lie below the surface of Oxfordshire.
In this sea would have lived crinoids, a marine animal that first appeared about 500 million years ago. They are sometimes referred to as sea lilies because some species have long stem-like stalks and flower-like heads. The hard parts of their bodies are primarily calcium carbonate (CaCO3).
When crinoids die, their skeletal remains, including individual pieces of the calcareous stalk, the head, and other parts, sink to the sea floor.
In the shallow marine environments, crinoid parts became mixed with other types of organic and inorganic material, such as shell fragments, sand, and silt.
Crinoid Head Crinoid Stalk
As more layers of sediment, including crinoidal fragments, accumulate on top of each other, the lower layers become compacted by the weight of the overlying material. Water seeping through the layers may contain dissolved minerals, particularly calcium carbonate. These minerals can precipitate, cementing the fragments together and forming solid rock. The continuous accumulation and cementation of these fragments gradually forms crinoidal limestone, which is a type of fossiliferous limestone.
Most of the gravestone is covered with lichens. The crinoids here are best observed at the white portion of the gravestone beside the bronze medallion where the limestone, and crinoids, are more easily seen.
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(zoomed in pieces of crinoid stalks also known as ossicles on a gravestone)
Resources:
Earle, S., 2019 Physical Geology - 2nd Edition, licensed under a Creative Commons Attribution 4.0 International License.
Johnson, C., Affolter, M. Inkerbrandt, P, and Mosher, C. 2017. An Introduction to Geology. https://opengeology.org/textbook/, Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Linsley, D., Devonian Paleontology of New York, 1994. The Paleontological Research Institution
Morgan, N. And Powell, P. 2015. The Geology of Oxford Cemeteries.
Powell, P. 2009, 2014 (reprint). The Geology of Oxfordshire,
Oxford University Museum of Natural History, Exhibition Gallery, “History of Life” and “Earth What can we learn about Earth from rocks and mineral beneath our feet?”.