Logging Tasks
1. Can you see individual mineral crystals without the use of a hand lens? Compare the texture with the texture of the nearby granite gravestone listed as an additional waypoint. Describe the differences.
2. Look at the BACK of the gravestone. Do you find additional reddish blotches here? How do they compare to the ones on the front of the gravestone? Are they significantly different or about the same? Point to one of them with your finger, pen/pencil or other object. Describe the size, shape and color.
3. You may be able to see individual ash grains but it will require a hand lens or, if you have a smartphone, you can use the photo app to get a magnified image. If you can find any ash particles describe, in general terms, their size, shape and color.
4. Look at the amount of wear you see. Compare with other nearby gravestones. In addition to the surface, compare how well or poorly the lettering is holding up. Is the Lake District Slate more or less resistant to weathering than other gravestones nearby? Why?
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. Luckily 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 Bullard gravesite at the posted coordinates. You are looking for a narrow uoright headstone with calligraphy beginning with "IN MEMORIAM BULLARD".
Types of rock
The three main types of rock are igneous, sedimentary and metamorphic.
How Metamorphic Rocks Are Formed
Metamorphic literally means to change (meta) form (morphos). Metamorhphic rocks are formed when (1) heat, (2) pressure and/or (3) chemically reactive fluids, change the original, or parent rock or protolith, into another type a rock. The parent rock may be an igneous rock, a sedimentary rock, or even another metamorphic rock.
Metamorphism takes place deep under ground. The deeper you go below the Earth’s surface the greater the temperature and pressure. Metamorphism occurs when solid rock changes composition and/or texture without mineral crystals melting (melting crystals would make it igneous).
Heat - an increase in temperature causes atoms in the mineral crystals to vibrate so vigorously that they jump around within the crystal, which remains intact. In other words, the changes are occurring while the rock is still solid.
Pressure - two types of pressure contribute to metamorphism. Confining pressure or lithostatic pressure is simply a result of rocks lying on top of each other. The pressure from above is balanced by pressure from below and from the sides. This may cause chemical reactions, just as heat does, to form new minerals.
Directed stress or differential or tectonic stress is an unequal balance of forces on a rock. It is generated by movement of lithospheric plates, the solid portion of the Earth made up of the crust and upper mantle. Unlike confining pressure, directed stress occurs at much lower pressures and does not generate chemical reactions that change mineral composition and atomic structure. Instead, directed stress modifies the parent rock at a mechanical level, changing the arrangement, size, and/or shape of the mineral crystals.
Chemically reactive fluids may also result in changes to the chemistry or mineralogy of crystals in the parent rock.
Lake District Slate
Perhaps one of the things that I find most interesting about geology is the stories told in the rocks. Identifying a rock specimen and putting a name to it is interesting. But the real fascination is unravelling the conditions in which it formed, where it was formed and the various processes going on around it.
Metamorphic rocks tell not just one story, but at least two. First, there are the circumstances that let to the formation of the parent rock. Then something happened to transform that parent rock into the metamorphic rock - another story. Here will unravel three stories - an ancient sea, volcanoes, and a mountain building event!
An Ancient Sea
About 450 million years ago, during the late Ordovician Period, the Lake District of what is now northern England, was a shallow tropical sea near the equator. Fine-grained sediments were deposited on the bottom of the sea forming mudstones made up primarily of clay and silt. Mudstones have the texture and composition of shale but are not as fissile, or easily split.
Volcanic Activity
Shortly after the deposition of mudstones in the ancient sea (Iapetus Ocean) began to close with the denser oceanic plate subducting, or slipping under the less dense continental plate. This resulted in an intense period of volcanic activity (Borrowdale Volcanic Group). Eruptions from these volcanoes resulted in ash being deposited on the sea floor.
The Big Squeeze
So now we have our mudstone and ash on the sea-floor, but we don’t have slate - yet! That will come about 50-60 million years later.
About 400-390 million years ago, during the Devonian Period, the remaining Iapetus Ocean closed with the collision of the Avalonia Plate (present-day England) and Laurentia Plate (Scotland and North America). This resulted in mountain building, faulting and regional metamorphism. The ash-laden mudstone, subjected to heat, and directed or tectonic stress metamorphosed the sedimentary rock to Lake District Slate.
Reader Bullard Gravestone
With this background, let’s now take a closer look at the Reader Bullard gravestone. We know that the slate used here was from stone that was formed from very fine particles of clay and silt that were deposited at the bottom of a shallow sea to form mudstone, which is very much like slate. Since the particles were so fine, what would we expect the texture of the gravestone to be like? Simply feeling the gravestone isn’t a very good test since the stone was very likely polished in order to make the gravestone. So instead take a very close look. Can you see individual mineral crystals without the use of a hand lens? Compare these to the crystals in the Florence Mary Seymour gravestone with the granite Celtic cross and base. It is 13 meters away at 126 degrees (N51 45.350 W 001 14.860)
Since we know that shortly after the deposition of mudstone, there was a lot of volcanic activity in the area. Ash was also deposited on the bottom of the sea. When rain fell during an eruption ash particles would clump together and fall as small reddish lumps. During burial they would be compressed. You can see some of these in the photo of the Bullard gravestone in this description.
Look at the BACK of the gravestone. Do you find additional reddish blotches here? How do they compare to the ones on the front of the gravestone? Are they significantly different or about the same? Point to one of them with your finger, pen/pencil or other object. Describe the size, shape and color.
Another sign of volcanic activity would be to find actual particles of ash on the slate. The sides of the gravestone are not quite as polished as the front and back surfaces. So you may be able to see individual ash grains but it will require a hand lens or, if you have a smartphone, you can use the photo app to get a magnified image. If you can find any ash particles describe, in general terms, their size, shape and color.
Because of its fine-grained texture, slate is very durable, weather resistant, and does not allow the growth of mosses and lichens. It also takes inscription very well. Look at the amount of wear you see. Compare with other nearby gravestones. In addition to the surface, compare how well or poorly the lettering is holding up. Is the Lake District Slate more or less resistant to weathering? Why?
Resources:
Bates, R.L. and Jackson, J. A. 1984. Dictionary of Geological Terms, The American Geological Institute
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.
Lake District National Park Authority, Education Service, Brockhole, Windermere, Cumbria LA23 1LJ Web site: www.lake-district.gov.uk Copyright waived for classroom use
Morgan, N. Gravestone Geology. https://www.gravestonegeology.uk/information_stone_types.php accessed 2024 01 26
Morgan, N. And Powell, P. 2015. The Geology of Oxford Cemeteries.
Powell, P. 2009, 2014 (reprint). The Geology of Oxfordshire
The Geology of Honister Slate Mines. 2016 https://honister.com/wp-content/uploads/2016/11/The-Geology-of-Honister.pdf
Woodhall, D G. 2000. Geology of the Keswick district. A brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 sheet 29 Keswick (England and Wales).