Meskó Csaba: Thermal Baths - Our Budapest (Budapest, 1999)

now Budapest emerged several times forming a stretch of land to sink repeatedly into the sea again, where, at the bottom, countless successive layers of alluvial rock sedimented one upon the other. Tectonic shifts in the crust resulted in faults and systems of faults, which allowed water to infiltrate the surface and move within the earth.

The limestone and dolomite deposited at the bottom of the Mesozoic sea rises at heights in excess of 400 metres above the Danube level in the hills of Buda. Two or three kilometres away from the easternmost of these hilltops, already on the Pest side, the same rock layers are a thousand metres below the Danube level. This means that while these layers account for the bulk of the hills rising in Buda, they provide but the bottom of the basin below Pest, a depression whose space is filled with significant layers of later sediments. Both limestone and dolomite produce, in themselves, impermeable formations, but due to their rigidity they easily break up and crumble in the wake of tectonic shifts. Over a sufficiently long period, the water moving in the cracks thus formed is capable of widening crevices into entire systems of interconnected passages. The process in which water dissolves limestone and dolomite to widen crevices into chambers is called karst formation. The end product is mostly the creation of caverns.

Cavernous limestone and dolomite is capable of storing large amounts of water in its interconnected cracks and passage systems. This water re-emerges above the surface in abundant springs, the so-called karst springs, at the end of its subterranean flow.

If the thousands of metres deep cavernous limestone and dolomite reaches to such depths that subterranean temperatures can heat up water stored in the cracks there, then the karst water, or part of the karst water, held in the cavernous masses of rock breaks out through thermal springs.

In the first, Eocene, part of the Tertiary period of the Cenozoic era, the region was covered by a shallow, tropical sea. At the bottom of this transparent, well-lit sea, calcareous algae thrived, together with the plenty of minute monoplastids which grew calcic skeletons. The petrified remains of these calcic skeletons and calcareous algae formed sheets of chalk covering older layers of limestone and dolomite over large areas. Due to its thickness and cavernous nature, this rock layer works as an efficient water conduit and reservoir. The system of interconnected fis15