Stone Sleuthing: The Starosel - Chetinyova Mogila

About 140 km southeast of Sofia, outside of the town of Starosel, Bulgaria, sits an impressive Thracian shrine known as the Chetinyova Mogila (mogila). Formed in the shape of a hill, the mogila has a grand view of the Pyasuchnik River Valley and is clearly the effort of powerful king. Some suggest it is the final resting place of Sitalkes, who during his brief life of 20 years extended the Thracian empire from the Danube River to the Aegean Sea. The historical details are uncertain, yet the magnificent dry stone construction remains in good shape after 2400 years of wear. My intention in visiting the Chetinyova Mogila was to approach its mysteries from a geological perspective. On first notice, I was impressed with the technical knowledge and workmanship that were required to construct it. About 300 stone blocks of decorative granite were used in the outer wall surrounding the earthen dome, the central staircase, and the hallway leading into the inner chambers. At roughly 250 kg each, the total weight of stone for the exterior construction equals 75 metric tons. As I admired the exterior work, one question loomed in my mind: where is the quarry for the granite stone blocks? Local stone seemed to be the obvious answer. With two colleagues, I identified the stone found along hillside road cuts and in quarries behind the shrine, but it was not the same. The local granites had a visibly light mineral texture, referred to as leucogranites. On the other hand, the mogila granite had a darker mineral texture, a greater content of the mineral plagioclase, and large pink crystals (phenocrysts) of alkali feldspar up to 1.5 inches in length to 0.5 inches in width. Based on these features, the mogila granite is described as a porphyritic granodiorite with large phenocrysts of pink alkali feldspar. In the stone trade, the mogila stone material is simply referred to as granite. To find its source, more field research was required. There is a saying that if you want to know the local geology – look to the walls of the towns. Working on this assumption, we continued our fieldwork along the outskirts of Starosel. We happened across chunks of the mogila granite haphazardly stacked in a yard, readied for the construction of a fence. Neighbours told us it was from the local quarries, the owner did as well, but it was clearly not so. When the neighbours had left, we stayed on to talk with the owner. Eventually he told us the stone had been taken from the remains of a farm storage building, not far away. Following his directions down a muddied road, we found the building. Its wall piers had been constructed with the same granite used in the mogila. The building had been raised during the communist period that ended in 1989. The mogila was built in the later half of the 4th century BCE and only recently unearthed in 2000. The storage building and the mogila had been built for very different reasons and during very different historical periods. Yet each structure shared a link to a common quarry found beyond the immediate area, but where? One of the professors engaged in the field research, hypothesized that rock from the Hisar pluton was the source of the mogila granite. During the Paleozoic Era, 310 – 340 million years ago, the Hisar pluton consisted of molten rock that cooled very slowly deep below the surface. In this environment, large phenocrysts of pink alkali feldspar could develop. We followed the lead and took the potholed road from Starosel to Hisar. Upon our arrival in Hisar, it was easy to choose a local wall for inspection. The Roman and Byzantine city/fortress wall of Diocletianopolis as it was called, stood 30 feet high in greeting. The massive wall was built with successive coursings of cemented stone and bricklayers. Close inspection revealed it contained porphyritic granodiorite with large phenocrysts of pink alkali feldspar – our mogila granite. In discussion with local residents, we discovered the source of the granite in an abandoned quarry in the Momina Banya quarter of Hisar. In ancient times, the Thracians shaped the granite rock into building blocks at the quarry. Typical blocks were fabricated to measurements of three feet in length and 1.5 feet in width and height. They were then transported by horse drawn cart to the site of the mogila some thirteen miles away as the road turns. Final fabrication of the stone was likely made on-site. Each block was finished with a pitched face; meaning laborers diligently squared the surfaces of the blocks with a hammer and chisel. Subsequently, the granite blocks were fit snuggly into place without the use of mortar and held together in key structural points with iron rods coated with lead. Part of the mystery had been solved. Our attention now turns to the soft, green stone that makes up the interior hallway and central chamber of the mogila. Known as an analcime tuff, Thracian stone masons precisely shaped, fitted, and installed massive blocks of it weighing as much as one metric ton each. The durable tuff was carved to display simple yet elegant details on the trim of the doorways. In addition, tuff blocks were fabricated and installed to form 10 Doric columns in the rounded central chamber, curved beams above the columns, and the central interior dome. Of particular interest is damage to the back wall of the central chamber. The grooved cuts into the tuff, coated with a mineral film (gypsum and anhydrite) suggest ancient vandalism. Was this damage the work of a Thracian king who ruled after Sitalkes? As we continue our inspection of the tuff blocks and search for the quarry, perhaps we can find the answer. Edward Monroe is conducting research in Bulgaria through a Fulbright Fellowship. His research project is entitled, “Building Stones of Bulgaria from Ancient Times to the Present”. The purpose is to create an educational field guide to Bulgaria’s unique historical stone buildings. Drs. Veselin Kovachev and Rossen Nedialkov from the Department of Mineralogy, Petrology, and Economic Geology of Sofia University, “St. Kliment Orhidski”, provide research support. Inquiries can be directed to ewmonroedc@hotmail.com