The Rila mountains are comprised mainly of ancient crystalline schists, interspersed with granites and gneiss and are part of the Rila-Rhodope Massif, the oldest landmass in the area. It was first uplifted during the Palaeozoic era, and subsequently on numerous occasions during the Tertiary period. However, the result of glaciation during the Pleistocene has provided today's alpine form of jagged peaks, cirques, deeply troughed valleys, and extensive moraines. The 'Blue Eyes of the Rila' are the most striking legacy of the period of glaciation, as almost 190 glacial lakes lie scattered throughout the mountains, with Ledenoto Ezero at the northern base of Mt. Musala being the highest glacial lake in the Balkan Peninsula.
The Rhodope is comprised of an ancient block of crystalline schists, limestones and granite, first uplifted during the Palaeozoic era and then subject of a long inactive period during which it was gradually eroded and levelled. At the end of the Tertiary is was again raised, but not as high as the neighbouring Rila and Pirin mountains and consequently escaped from glaciation. The geology of the western and eastern sections of the Rhodope is markedly different. In the west there is a typical karst terrain of limestone rocks pitted with caves and scored with deep gorges, whereas in the east much younger igneous rocks give rise to an interesting landscape of wind and rain weathered rock formations in the shape of mushrooms and pinnacles. Also, the Rhodope have significant commercial mineral deposits, particularly of copper, iron, lead and zinc.
In the Pirin, especially in the northern zone, the base rock structure of limestone and of granite changes frequently and very abruptly. An excellent example of this is seen on the Vihrenski Preslap saddle. Also, the Pirin show extensive examples of glaciation effects although there are no longer any glaciers present i.e., the alpine-like narrow crested peaks, scooped-out cirques and deeply troughed valleys. A particularly noteworthy feature in the valley floor at the base of the northern section is the abundant evidence of subterranean volcanic activity that gives rise to numerous hot mineral springs. Many of these hot mineral springs have a long history of being highly regarded for their curative properties, especially for eye and skin ailments. This feature has resulted in the establishment of numerous spas, many dating back through antiquity. In Banya village, which translates as meaning 'bath', there are substantial remains of Roman and Turkish baths. There are over 70 separate mineral springs in Banya, each of which produces its own constant stream of hot mineral water at 57°C. One of these springs is used to fill the outdoor Olympic-sized swimming pool and associated children's pool, whereas another is used to provide the water in the local public baths.