I step off the marble curb of my hostel into the quiet streets of Plaka in Athens, Greece, and I am alone for a just a moment. The same grey and black cobblestone streets that were alive with the energy of tourists and vendors only hours before are now quiet. Once bright pastel buildings have been dulled by the sun’s rays. Their shutters hang askew, and clothes lines overhang the narrow streets like banners stretched between buildings. Next door, an old man sweeps the streets outside the Taverna with its red and white checkered table cloths.
It is in this moment that it hits me. It’s the same feeling of excitement and anticipation that I experience each time I wake up in a foreign country about to begin my next adventure. This time it’s different though, there is a larger goal in play, and I am not just here to experience another country and its culture.
Today we begin our first field work exercise in geology. I have been an engineering major for the last few years, but I am still unsure of the degree and want to discover if geology is my passion. The rest of my group is now filing out behind me and we are off. As we emerge from the narrow European streets I pinpoint our destination, and the source of my excitement from the last few days.
As I walk along the cobblestone streets I see rising up from the center of Athens a rather unique hill (a klippe). The Acropolis sits with six others like it in a topographic basin that stretches several miles from the (northeast) to the (southwest) before falling into the Aegean Sea. I’ve looked up at the 70-meter cliffs for the last couple of days as I have explored Plaka, but it isn’t until I am walking along its 300-meter base that I realize how large and unusual it is to have these sporadic hills throughout the basin (see figure 1).
Figure 1: A map of the topographic basin and surrounding area. The basin is outlined in black and surrounding the basin are four mountains. The red point is the Acropolis. The green, pink, and blue points are three of the major klippes in the basin.
Perched on top of the Acropolis is one of the most architecturally and artistic man made structures from Ancient times, the Parthenon. I am excited to experience the history and walk in the steps of people from 430 B.C. (when the Parthenon was built). Archaeologists have evidence that suggests people have been living around the Acropolis since 6000 B.C., and to me it is fascinating to experience such history.
Before reaching the marble staircase of the Acropolis I climb along a dirt path that takes me through the geologic layers. I have been tasked with photographing the different types of rock that I see and describing them as my first field work assignment. I am excited to begin walking through geologic history to discover what has happened to this basin in the last 100 million years and to solve the mystery of these klippes.
As I walk the path, the first rock that I come to happens to be the one I am standing on and is a reddish pink colored rock that is made up of consolidated material, meaning it is not loose (see figure 2). I notice that the rock has a “platy” look to it due to the folds in the rock. From class discussions I know that this is rock is the Athenian “Schist”. I also know that this 72-million-year old rock is not really a schist, but is a sedimentary rock composed of shale (mud) and sandstone with minor conglomerate that was lightly metamorphosed before the Alpine Orogeny.
Figure 2: The first layer in the Acropolis is the 72-million-year old Athenian “Schist” (pictured above).
Moving further up the path is only minutes in present time, but is 22 million years in geologic history. It’s fascinating to me how I can walk through millions of years and the rocks tell me the story of what was happening at that time. Using the diagram from class of the geologic layers (figure 3) I know that walking the 22 million years through time will bring me to the layer above the Athenian “Schist”. This rock is the Cataclastic Limestone (fault breccia) that formed 50 million years ago. It is a rather thin layer in the rock record compared to the Athenian “Schist” below, and Limestone that sits on top.
Figure 3: The stratigraphic layers under the Parthenon. From bottom to top is the Athenian “Schist” (72 ma), Cataclastic Limestone (fault breccia, 50 ma), and Limestone 100 ma. The normal faults are the dark lines that are nearly vertical. The less dark lines that separate the Athenian “Schist” and Cataclastic Limestone are the reverse thrust fault that moved the Limestone on top of the Athenian “Schist”.
It was the layer that the Parthenon was built on, limestone, that shear amazement overcame me. Limestone is a chemical sedimentary rock that is composed of calcium carbonate, sand, silt, and clay and is formed in oceanic environments. The limestone here was formed in the Tethys Sea between Gondwana and Laurasia 100 million years ago.
There was nothing particularly unusual until I started looking at the ages of each of the rock types. The limestone that the Parthenon was built on sits on top of the 50-million-year old fault breccia and the 72-million-year old Athenian “Schist” that is insitu (in its original place). That was when it struck me that meters thick of one rock had somehow ended up on top of two other rock types that were between 30 and 50-million-years younger.
I wondered what sort of process could put a much older rock on top of a much younger rock. Through the timescale development of each of the layers the mystery of the layer development was solved. The limestone was deposited first in the Tethys Sea, then the Athenian “Schist” was deposited and slightly metamorphosed, and then the limestone was metamorphosed and buried. The next event, the thrust of the limestone 120 km onto the “schist”, is how the limestone, an older layer, ended up on top of the “schist”, the younger layer.
The limestone was pushed by a thrust fault (a low angle reverse fault) due to continental collision that occurred during the Upper Eocene orogenic phase. This process can be assimilated to a large rock vessel being run aground and moved inland. The thrusting that brought the limestone to where it is today in geologic terms is called a klippe.
In this thrust fault process, the limestone was the hanging wall and the Athenian “Schist” was the foot wall (see figure 4). The foot wall is pristine compared to the hanging wall because the hanging wall gets busted up through its movement upwards, while the foot wall remained intact as it dropped down.
Figure 4: Pictured above are the examples of a normal fault, reverse fault, and thrust fault. In a normal fault the hanging wall moves down relative to the foot wall. In a reverse fault the opposite occurs, the hanging wall moves up relative to the footwall. A thrust fault is just a low angle reverse fault and the footwall typically remains in place while the hanging wall is pushed on top.
The force required to move the limestone created enormous amounts of pressure and heat between the Athenian “Schist” and the limestone, and it was through this process that the fault breccia (cataclastite) was created. To think of the magnitude of force that was required for these processes to occur and to move the limestone 120 km on to the Athenian “Schist” was mind blowing.
As I continued along the dirt and rock path I reached the base of the steps to the Parthenon, I was awed that I was able to visit a place that held so much history and had the extent of architecture that it did. The white marble steps were smooth and slick after having been walked on for two and a half thousand years. The marble arch that rested at the top of the marble stairs was yellowed with weathering and had fragments missing. The black clouds just off in the distance brewed in the background with the approaching storm.
As I moved through the arch, on my right was the Parthenon, and to my left the Temple of Athena. Just beyond the Temple of Athena I could see one of the six other klippes in the topographic basin. When the limestone was pushed into this area it was one large sheet, so I wondered how only a few hills remained. After some research I found a paper that had evidence to show a large river having moved through the area, and as it moved the limestone sheared off at the normal faults (see figure 1) and was broken down and eroded away to what we see today (see figure 5).
Figure 5: Overlooking Athens to the northeast from the top of the Acropolis is a second klippe. This one is slightly bigger than the Acropolis and in Figure 1 is the green pin.
Standing atop the Acropolis and looking out over Athens to the sea I had yet another question and wondered why Pericles in 460 B.C., and those for thousands of years before him, had decided to build and live around the Acropolis. After a little research, I found that a number of reasons made the Acropolis the ideal klippe to build on and around. The Acropolis is one of the highest, but not the highest hill, in the basin. Its location nearest to the sea makes it unique.
People were probably drawn to the Acropolis as early as 6000 B.C. because of the nature of the rock that it is composed of. The top layer of limestone and the cataclastite limestone just below are both extremely permeable. Cataclastite is permeable due to the large clasts inside the rock that make it more porous and allow water to travel through. This movement of water through the rock allows springs and caves to develop making it the ideal location for civilization to form.
It is likely Pericles and those before him built on top of the Acropolis because there was visibility of the sea to the south and visibility in all directions for an approaching enemy. One of the most important aspects of the Acropolis though is that it is inaccessible from all sides. The exception is from the south side, which had stairs leading up. This meant the Acropolis acted as a safe haven for citizens when their cities were under attack.
As I sit atop the Acropolis amongst the ruins, I have the opportunity to think about the journey I’ve taken through time geologically and historically. A 100 million years ago limestone was deposited in the Tethys Sea and 20 million years later the Athenian “Schist” was deposited. Through additional processes of burial, metamorphism, and thrust the klippes that are seen today are the result.
These processes are what enabled humans to move into this area because the limestone caves provided shelter and springs from the limestone provided water. This combination made development ideal. I have really enjoyed this exercise because it tied together the geology of the Acropolis and the human history that has been interwoven into it. With more field experience in the next couple of weeks and exploring the geology of Santorini I am hoping to come closer in making my decision in pursuing either the engineering degree or geology degree.
Regueiro, M., M. Stamatakis, and K. Laskaridis. “The Geology of the Acropolis (Athens, Greece).” European Geologist 38 (2014): 45-51. European Federation of Geologists. Web. 8 June 2017.