Our goals for summer 2016
Our main goal for “Team Stoa” this summer was to relocate and identify, map, rock-type, and record as many of the 2,400+ blocks listed in the Stoa database on filemaker. In the field, we used the block charts created in 1991 to aid in our quest to locate the actual blocks. What we quickly discovered was that the block fields to the west of the Stoa have been significantly less disturbed that those to the east, because blocks have been moved on the east side to create a path for tourists.
The heroes of our story
Equipped with measuring tapes, sharpies, clipboards, and 25-year-old maps (and a better grip on sanity than we ended with) four of us set out to take on the Stoa. We used a combination of measurements taken when the blocks were excavated, found in the block books, and more updated measurements already entered in the database.
However, identifying, labeling, and mapping the blocks was only half the battle; additionally, we wanted to determine the type of rock of which each identified block is composed. Collecting the data allowed us to observe a correlation between vugginess of dolomitic limestone (DL) and decorative detail. Mostly, blocks with architectural detail are made of a more massive, light tan dolomitic limestone, while a vuggier, light grey dolomitic limestone is the material for more general blocks. There are some exceptions.
In order to determine the type of rock we did visual inspections of the blocks, noting the color, surface features (e.g. vugginess, calcite deposits), and fossils. The most diagnostic test we did to differentiate between the dolomitic limestones and limestone and other types of rock was to drop diluted hydrochloric acid on to the rocks. In dolomitic limestone, there is a less vigorous reaction with the HCl because the principle mineral is dolomite (CaMg(CO3)2), as opposed to calcite in limestone. The acid does not react as much with the magnesium in dolomite as it does with the calcium in calcite.
The Stoa team also took a brief hiatus from our beloved block fields to show some love for the Stoa itself and to hone our “vigourous sweeping” skills. We cleaned out the dowel holes, swept off the foundations, and uncovered the inner side of the eastern foundations along approximately half the length of the building. The purpose of this cleaning (other than building create forearm muscles) was to allow Michael to take publication quality aerial photos of the building with his drone.
- Leah Neiman, August 2016
Samothrace has a variety of rocks that were used by the ancient Greeks in their construction, including in the “Sanctuary of the Great Gods”.
The origin of these rocks reflects the sometime violent history of the island. The southern margin is truncated by the long-active Anatolian Fault. Major earthquakes and vertical upheaval of the sea floor have tectonically juxtaposed rocks of different geologic origins. One of the more interesting rocks in the Sanctuary is called pillow basalt (Fig. 1).
This rock seems to have been revered by the ancients and is seen as a sacred altar stone in the Sanctuary. Pillow basalt only occurs as sea floor eruptions throughout the world’s ocean floor and is considered one of the main components of the crust of the earth. On Samothrace these rocks are dated as Jurassic in age (200-145 mybp). Nearer to the present time, large intrusions of light-colored granite magma shot through the basalt producing spectacular outcrops (Fig.2).
These granites are dated at Late Oligocene (34-23 mybp) and were produced by the collision of major tectonic plates is this region that partially re-melted existing rocks producing granite magma. These granites are the second most abundant rock making up the crust. Therefore, Samothrace displays both of the major components of the of the crust of the Earth.
- Dr. William Size, 2016.
Figuring out rocks?
Use hydrochloric acid.
Fizz! Oh, a limestone…
Acid testing is crucial for identifying various stone types in the field. Often, rocks look the same, but are quite different. Let’s use limestone as an example. Dolomitic fossiliferous limestone and fossiliferous limestones look rather similar. However, a chemical difference exists. Fossiliferous limestone, as we define in our stone maps, has a high percentage of calcite, or calcium carbonate (CaCO3), where as dolomitic fossiliferous limestone, or CaMg(CaCO3)2, has more magnesium present, replacing some of the calcium. Due to the slight variation in chemical properties, fossiliferous limestone will react instantaneously with hydrochloric acid (HCl), releasing carbon dioxide (CO2). This reaction can be seen in the form of a fizz; bubbles will start forming on the stone. In contrast, dolomitic fossiliferous limestone will have a delayed reaction when presented with HCl, and won’t fizz as violently. Therefore, HCl is a great way to test for dolomitic qualities in limestones.
The reaction of calcite with hydrochloric acid:
CaCO3 (s) + 2HCl (aq) –> CO2 (g) + H2O (l) + Ca+ (aq) + 2Cl– (aq)
Art History Department
581 S. Kilgo Circle133 Carlos Hall
Atlanta, GA 30322
Bonna D. Wescoat, archaeology
William Size, geology
Vicki Hertzberg, statistics
Michael Page, geography
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