Lying silently at the foot of Mount Errigal, the tallest of the Derryveagh Mountain range and the largest in Donegal, is the stunning ruin of the Old Church of Dunlewey. A beautiful building which is a lasting memorial to a great love affair between James Russell, the landlord of the Dunlewey estate, and his wife Jane.
James Russell married Jane Smith in 1825 and they set up married life in London. James would go on to make a fortune as a hop merchant. The couple were blessed with five daughters: Fannie, Henrietta, Emma, Louisa Sarah, and Josephine.
Using the wealth they had acquired in London, the couple decided to leave England and purchase the Dunlewey estate, an area of sheep, lakes and outstanding natural beauty with the wild Atlantic to the west and the Derryveagh Mountains to the East.
The estate is recorded as consisting of a house, outbuildings, gate lodges and 3825 acres of land, a large holding by the standards of the day.
Little is written of their time in Dunlewey, but James died on the 2nd of September 1848. Heartbroken, his widow decided to build the church as a monument for her husband.
Like the Taj Mahal, the Church of Ireland building was constructed using marble and also quartzite which was quarried locally.
The supply of marble in the nearby quarry has now been depleted. The red brick in the arches of the windows was produced locally. Remnants of the brickfield are still visible near Oilean Ghrainne when the level of the lough is low.
A simple EarthCache giving a little geology lesson and something to feast your eyes on.
What is Marble?
Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. Under the conditions of metamorphism, the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals.
How Does Marble Form?
The transformation of limestone into marble usually occurs at convergent plate boundaries where large areas of Earth's crust are exposed to the heat and pressure of regional metamorphism. Some marble also forms by contact metamorphism when a hot magma body heats adjacent limestone or dolostone. This process also occurs at convergent plate boundaries
Before metamorphism, the calcite in the limestone is often in the form of lithified fossil material and biological debris. During metamorphism, this calcite recrystallizes and the texture of the rock changes.
In the early stages of the limestone-to-marble transformation, the calcite crystals in the rock are very small. In a freshly-broken hand specimen, they might only be recognized as a sugary sparkle of light reflecting from their tiny cleavage faces when the rock is played in the light.
As metamorphism progresses, the crystals grow larger and become easily recognizable as interlocking crystals of calcite. Recrystallization obscures the original fossils and sedimentary structures of the limestone. It also occurs without forming foliation, which normally is found in rocks that are altered by the directed pressure of a convergent plate boundary.
Recrystallization is what marks the separation between limestone and marble. Marble that has been exposed to low levels of metamorphism will have very small calcite crystals. The crystals usually grow in size and as the level of metamorphism progresses.
Clay minerals within the marble will alter to micas and more complex silicate structures as the level of metamorphism increases. Some might contribute to the formation of gems. Marble is usually a light-colored rock. When it is formed from a limestone with very few impurities, it will be white in color. Marble that contains impurities such as clay minerals, iron oxides, or bituminous material can be bluish, gray, pink, yellow or black in colour.
What is Quartzite?
Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz. It forms when a quartz-rich sandstone is altered by the heat, pressure, and chemical activity of metamorphism.
Metamorphism recrystallizes the sand grains and the silica cement that binds them together. The result is a network of interlocking quartz grains of incredible strength.The interlocking crystalline structure of quartzite makes it a hard, tough, durable rock. It is so tough that it breaks through the quartz grains rather than breaking along the boundaries between them. This is a characteristic that separates true quartzite from sandstone.
Quartzite is usually white to gray in colour. Some rock units that are stained by iron can be pink, red, or purple. Other impurities can cause quartzite to be yellow, orange, brown, green, or blue.
Where Does Quartzite Form?
Most quartzite forms during mountain-building events at convergent plate boundaries where sandstone was deposited on a continental plate. There, the sandstone is metamorphosed into quartzite by the intense pressure of a plate collision and often by deep burial. Compressional forces at the plate boundary fold and fault the rocks and thicken the crust into a mountain range. Quartzite is an important rock type in folded mountain ranges throughout the world.
1) Identify the Marble, Do you think it was formed from a limestone with very few impurities or lots of Impurities? Give a reason for you answer.
2) Take a closer look at the marble, what size are the crystals. Do you think it has been exposed to high or low level of metamorphism?.
3) Find the Quartzite. Has it been stained by iron or some other impurities? Give a reason for your answer.
4) I like to see photographs in my logs so please post a pic
Of yourself or GPSr at ground zero (optional).
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