Source: Own photos

Geology of Derry Walls

Derry is truly a unique city in many ways. Walking along the walls of Derry, we are not only dealing with an exceptional structure. It is truly rare in Europe to find walls that have survived virtually untouched for centuries until present times. These walls can tell us many stories. As this is an Earthcache, let's try to decipher the geological tale of these walls. Personally, during my stay in Derry, I completed several full laps walking along the walls of Derry. At first, I gathered a lot of historical facts about their construction. Then, I observed the walls and their individual stones for a long time. The longer I observed, the more intrigued I became by these walls. 

The walls were built in the early 17th century. It seems to me that this was done in quite short time, considering the circumstances. It took only 5 years to build very thick and high city walls which are more than 1 kilometer long. Considering that there were no advanced construction technologies, machinery or infrastructure that we have today, it must have been an impressive construction project. 

Looking at the walls, regardless of the location, we immediately notice that it has been built with relatively small stone fragments. One could say it is constructed with stone rubble. The size of the individual stones used in the construction also testifies to the incredible craftsmanship of the builders. The facades of the walls are very intriguing. It seems that each stone has been properly chosen and placed exactly where it fits. Each stone has found its proper place depending on its shape. 

Walking along the walls and contemplating how and with what they were built, I realized that the key to all of this is, after all, geology. The geology of the region where the city of Derry is located. Because the stones for such a large construction project could not have been brought in such large quantities from quarries quite far away. Therefore, the way the walls were built and which material was used can tell us a lot about the geology of the region. Pondering over this, I realized how rich the geology must be, how great the geological diversity of this part of the island is. I encourage you to walk around the entire perimeter of the walls of the city of Derry. However, to complete this Earthcache, it is enough to examine the walls in one place. That place is near the Tower Museum and the corner of the walls with the stairs leading to the walls. 

At the ground zero location of the Earthcache we will find all three main types of rocks. In the listing, I will describe these three types of rocks one by one. Your task will be to make observations, identify where the different rock types have been used and draw conclusions from observations.

In geology, we distinguish three main types of rocks: plutonic, sedimentary, and metamorphic.

Igneous Rocks

Igneous, or also known as magmatic, rocks are formed when hot, molten rock called magma solidifies and crystallizes. Igneous rocks are divided into two groups:

1. Extrusive, volcanic rock is formed when magma rises and cools above the Earth's surface or very close to the Earth's surface. Lava cools and solidifies almost instantly when exposed to relatively low atmospheric temperature. Rapid cooling means that mineral crystals do not have much time to grow, so these rocks have a very fine-grained or even glassy texture. Bubbles of hot gas are sometimes trapped in lava and form vesicles.

2. Intrusive igneous rocks, also known as plutonic igneous rocks, are formed when magma is trapped deep inside the Earth. They cool very slowly over thousands or millions of years until they solidify. Slow cooling means that individual mineral grains have a very long time to grow, so they grow into relatively large sizes. Intrusive rocks have a coarse-grained texture.

Sedimentary Rock 

Sedimentary rocks are formed from pre-existing pieces of rocks or once-living organisms. They are created from sediments that accumulate on the Earth's surface. Sedimentary rocks are a type of rock that can also contain fossils.
Sedimentary rocks can be divided into many different categories. For simplicity, let's summarize two categories:

1. Clastic sedimentary rocks: Clastic means they are composed of small pieces (clasts) of pre-existing rocks. These rock fragments are formed when various types of rocks undergo erosion. These pieces are transported into a water reservoir where they accumulate. When the sediment is buried deep, it compacts and forms sedimentary rock. The particle size ranges from microscopic clay to massive boulders. The names of rocks are derived from their grain sizes. Grain size can be microscopic, invisible to the human eye, it can be small but visible within the range of up to 2 millimeters, such as sand, or larger than 2 millimeters composed of small pebbles and forming conglomerate.

2. Biogenic sedimentary rocks: Biogenic sedimentary rocks are formed when a large number of living organisms die. These can be plants or animals, or both. Chalk is an example of this type of rock. In this way, limestone can also be formed when remnants of living organisms accumulate at the bottom of a water reservoir. Coal is another example of this type of rock.

Metamorphic Rock 

Metamorphic rocks, or transformed rocks, were originally a different type of rock but have significantly changed from their original state. The processes that cause this change involve exposure to high heat (above 200 °C), high pressure, or a combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet. The process of rock metamorphism does not involve melting, but rather transforms and causes recrystallization. If rocks melt, they are considered igneous rocks.

1. Foliated metamorphic rocks: Some types of metamorphic rocks are characterized by foliation. Foliation refers to the parallel arrangement of certain mineral grains, giving the rock a striped appearance. Foliation occurs when pressure compresses flat or elongated minerals in the rock, causing them to align. This helps identify the direction in which the pressure was applied. Individual layers of foliation can be easily observed as they have different colors of minerals.

2. Non-foliated metamorphic rocks: Non-foliated metamorphic rocks have a uniform structure. The absence of foliation can occur for several reasons: the rock is subjected to uniform pressure, there is an abundance of the same minerals, and the absence of platy minerals such as sheet silicates. Metamorphic rocks can form in several ways. Some rocks, such as limestone, are made up of minerals that are not flat or elongated. Regardless of the magnitude of the applied pressure, the grains do not align! Another type of metamorphism, contact metamorphism, occurs when hot igneous rock intrudes into pre-existing rock. The pre-existing rock is essentially "baked" by the heat, causing a change in the mineral structure of the rock without the addition of pressure.

Questions: 

1. Look at the wall at Derry Wall at ground zero, where are you able to identify:
a) ignous rock
b) sedimentary rock
c) metamorphic rock

2. Which rock type of the three above has been used in the largest quantities to build the Derry Walls?

3. In the listing I have described two subtypes of each of the rock type. Once you have identified the rock type from question number 2. try to identify the subtype. 

4. Considering the fact that the Derry City Walls have been build at the begining of the 17th century, assuming that transport from far away of large amount of building stone at time was not possible what conclusions can we draw about the geological structure of Derry and its vicinity?

5. Optional question: what is the exact name of the rock used in largest quantities to built Derry Walls?

Sources:

https://opengeology.org/petrology/
https://geoschol.com/counties/DERRY_GEOLOGY.pdf
https://stonedatabase.com/building/derry-walls/
https://stonedatabase.com/natural-stone/derry-schist/