Cirque morphometry of Rila and Pirin Mountains (Bulgaria)

This study presents a morphometric analysis of cirques based on digital terrain models in the Rila and Pirin Mountains of Bulgaria, addressing a research gap for the northeastern Balkan Peninsula. Cirques, key indicators of past glaciation, vary widely in shape and size, influenced by bedrock, topography and (paleo)climate. Beside classical cirque characteristics this study utilises additional morphometric parameters, including cirque volume, depth and a simple slope-based categorization. The analysis focuses on comparing the two mountains, which share similar elevations but differ in geology and ridge orientations. Results indicate that cirque size parameters follow a lognormal distribution. Further on, morphological differences between the cirques of the two ranges are explored, with the Rila cirques generally being shallower and less developed, likely due to the presence of extensive planation surfaces. In contrast, the Pirin cirques, particularly those formed on marble bedrock, are more incised. The aspect distribution of cirques shows a clear concentration, in the case of Pirin towards the NE, in the case of Rila towards the N, which can be attributed to the NW-SE and W-E strike of the main ridges in the two ranges, respectively. The cirque floor elevation of the Rila and Pirin cirques is among the highest in the Balkans. The research suggests that cirque development in this region was influenced by a combination of continental climate, limited precipitation, and prevailing westerly winds that influenced snow accumulation. The findings contribute to the understanding of glaciation dynamics in the Balkans, emphasizing the role of pre-existing topography and climatic conditions in shaping glacial landforms. • Rila and Pirin are the highest mountains in the Balkans with 318 cirques together. • Cirque aspect main directions: NE (Pirin), N (Rila) controlled by ridge orientation • Cirque floor elevations: the highest in the Balkans suggesting limited precipitation • Cirque size (area, volume, depth) follows lognormal distribution. • Geology and planation surfaces influence cirque development.