LOGGING TASKS
Which is coarser-grained, the rock used for the cross or the rock used for the plinth? Which cooled more quickly? How do you know?
Measure the largest feldspar crystal you can find. How big is it? What is its general shape i.e. circular, rectangular, irregular? Is it on the cross or the plinth? Why do you think the larger crystals are found on this part of the gravestone?
Include a photo pointing to a crystal of the mineral biotite. Is biotite a type of quartz, feldspar, or mica?
NOTE: There are additional questions in the description below but these are only to guide you through your observations. The above Logging Tasks are all that is required for this Earthcache. Think of the others as questions to ask yourself, not questions that need to be answered out loud. 
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 one of the gravesites for this EarthCache. But you are certainly encouraged to allow yourself time to explore the rest of this fascinating cemetery.
The "Trailhead" 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 40 meters to the Florence Mary Seymour gravesite at the posted coordinates. You are looking for a Celtic cross on a rough hewn base.
Types of rock
The three main types of rock are igneous, sedimentary and metamorphic.
How Igneous Rocks Are Formed
Igneous rock is formed when liquid rock cools and becomes solid rock. This molten material is called magma when it is in the ground and lava when it is on the surface. Only the Earth’s outer core is liquid; the Earth’s mantle and crust is naturally solid. However, there are a few minor pockets of magma that form near the surface where geologic processes cause melting. It is this magma that becomes the source for both volcanoes and igneous rocks. It is the latter that we will explore here.
Classification of Igneous Rocks
Igneous rocks are classified based on texture and composition. Texture describes the physical characteristics of the minerals, such as grain size. This relates to the cooling history of the molten magma from which it came. Composition refers to the minerals that make up the rock and its chemical composition. Cooling history is also related to changes that can occur to the composition of igneous rocks.
Adapted from An Introduction to Geology, Chapter 4 Igneous Processes and Volcanoes
Texture
If magma cools slowly, deep within the crust, the resulting rock is called intrusive or plutonic. The slow cooling process allows crystals to grow large, giving intrusive igneous rock a coarse-grained or phaneritic texture. The individual crystals in phaneritic texture are readily visible to the unaided eye.
When lava is extruded onto the surface, or intruded into shallow fissures near the surface and cools, the resulting igneous rock is called extrusive or volcanic. Extrusive igneous rocks have a fine-grained or aphanitic texture, in which the grains are too small to see with the unaided eye. The fine-grained texture indicates the quickly cooling lava did not have time to grow large crystals.
Composition
Composition refers to a rock’s chemical and mineral make-up. For igneous rock, composition is divided into four groups: felsic, intermediate, mafic, and ultramafic. These groups refer to differing amounts of silica, iron, and magnesium found in the minerals that make up the rocks. It is important to realize these groups do not have sharp boundaries in nature, but rather lie on a continuous spectrum with many transitional compositions and names that refer to specific quantities of minerals.
I've highlighted the column for granite in the figure below.
Adapted from An Introduction to Geology, Chapter 4 Igneous Processes and Volcanoes
Granite
Granite is a commonly-used term, often referring to things that a geologist would not call granite. By definition granite is a course-grained, felsic, intrusive rock. So let's break that down.
It's coarse grained, or has a phaneritic texture, because it cooled slowly, deep in the crust, giving the crystals time to grow. Because it was deep down and never made it to the surface before cooling we refer to that as intrusive. So that takes care of the texture.
Now on to the composition. We are defining granite as feslic. Felsic is geological shorthand for Feldspar + Silica. It generally refers to a volcanic rock with a high silica composition. In the case of granite the presence of feldspar (both potassium feldpspar or orthoclase, and plagioclase feldspar) and quartz (crystalline silica) make it felsic and contribute to the lighter color of felsic rocks. Some darker minerals may also be found in granite such as the dark biotite mica.
The presence of quartz is a good indicator of granite. Granite commonly has large amounts of salmon pink potassium feldspar and white plagioclase fledspar crystals. Granite is a good approximation for the continental crust, both in density and composition. In fact, granite and granitic rocks are up to 70-80% by volume, of the Earth's contintnental crust.
Florence Mary Seymour Gravestone
A Tale of Two Granites and Two Origin Stories
We are fortunate to have two different types of granite at one gravestone. They illustrate two of the ways that granite can be formed. They are both the result of orogenies (mountain building events). They are both associated with batholiths, large bodies of of intrusive igneous rock, usually granite, that form deep below Earth’s surface as magma slowly cools and crystallizes. This process takes millions of years and results in coarse-grained rock.
The granite for the Celtic cross may have come from quarries in Galway, in the Republic of Ireland, that are part of the Galway Granite Batholith. The Caledonian Orogeny (490-390 mya) occurred due to the closure of the ancient Iapetus Ocean and the collision of Laurentia (proto-North America) and the micro-continents, Baltica and Avalonia. The denser oceanic plate, subducted, or slipped under, the more buoyant continental plate. This created magma through the partial melting of the mantle and lower crust above. Under these circumstances the magma was silica rich. Granite was intruded into the thickened crust. The intrusions occurred over a period of 30 million years but the main period occurred approximately 400 million years ago.
The result - coarse grained, felsic (silica-rich), intrusive rock - granite!
The plinth, or base, is probably from Cornwall which is part of the Cornubian Batholith, the large mass of granite that forms much of the peninsula of SW England. After Laurentia had converged with Avalonia and Baltica, bringing the two halves of the UK together in the Caledonian Orogeny (370-290 mya), the ancient Rheic Ocean closed resulting in another collision of tectonic plates. But this time it was two continents (Gondwana and Laurussia) that collided. This resulted in thickening of the crust and widespread melting of the lower crust (rather than the subduction related melting of the Caledonian Orogeny). Large masses of granite intruded into the overthickened crust. This yielded coarse-grained, silica-rich granites due to slow cooling of magma with the crust.
And once again we have coarse-grained, felsic (silica-rich), intrusive rock - granite!
Texture and Composition of Seymour Gravestone Granite
If we want to confirm that this gravestone is granite we should be looking at texture and composition.
For texture, we would expect it to be coarse-grained since granite is intrusive, allowing time for crystals to grow. But you will also be comparing different parts of the gravestone.
For composition, we know that granite is felsic. So we should be looking for feldspar, and silica, in the form of quartz. We may also find darker minerals like biotite.
So first, take a close look at the Celtic cross that forms the upper potion of the gravestone. Check the texture. Is it fine-grained, medium-grained or coarse grained? If its fine-grained, you won't be able to see individual crystals. It wil have a smoother texture. If it's coarse-grained you should be able to see individual crystals quite clearly, without a hand lens or other means of magnifcation. Medium-grained would fall in between.
Then examine it for composition. If you're lucky, and it's a sunny day in Oxford, you may catch glimpses of mica refelcting the sun. Then look for light colored minerals. The more abudant, white minerals are feldspar, while the smoky gray ones are quartz. Don't worry if the feldspar is hard to see. I will guide you to some that are easier to spot in just a minute.
Now look at the plinth, or the base of the gravestone.
Check the texture. Does it seem more, or less coarse then the granite above? You may find that the crystals are larger here. That should tell you something about the relative cooloing rates of the two different rocks used here.
The very large rectangular crystals are feldspar. The dark minerals are biotite and iron-rich form of mica. Again, check to see if they sparkle are you look from different angles. But even if they don't, they should still be easy to pick out from the lighter background minerals.
After making these observations and reading the description you should have all of the information necessary for the logging tasks.
Resources:
Bates, R.L. and Jackson, J. A. 1984. Dictionary of Geological Terms, The American Geological Institute
Cornubian Batholith. https://variscancoast.co.uk/cornubian-batholith, accessed 10 Jan 2025
Earle, S., 2019 Physical Geology - 2nd Edition, licensed under a Creative Commons Attribution 4.0 International License.
Galway - County Geological Site Report. https://gsi.geodata.gov.ie/downloads/Geoheritage/Reports/GY124_Shannapheasteen_Quarry.pdf, accessed 10 Jan 2025
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.
Morgan, N. And Powell, P. 2015. The Geology of Oxford Cemeteries.
Plate Tectonics of the UK. https://www.geolsoc.org.uk/Plate-Tectonics/Chap4-Plate-Tectonics-of-the-UK/, accessed 10 Jan 2025
Powell, P. 2009, 2014 (reprint). The Geology of Oxfordshire