Hamvas Cave

Hamvas Cave is among the specially protected caves of Hungary. It is located in the Vértes Mountains in the Danube–Ipoly National Park, in Csákvár.

Location and extent of the cave

In the DK part of the Vértes Mountains, on the outskirts of Csákvár, on the slope of Gánt, in the immediate D-side neighbourhood of Gánt, the cave lies in a bauxite mine that has been reclaimed to the present day. The entrance of the cave is in the spoil wall of the former bauxite extraction of the Long‑haraszt, which is located on the DNy side of Gém‑hill. The cave is most easily approached from the Gánt D side. From the monument, one must go about 0.6 km on the road leading to the reclaimed bauxite mine (this distance can also be reached by vehicle), then walk another half kilometre diagonally eastward on a slope carved by water flows, overgrown with brambles, to the mine that houses the cave.

The spacious entrance of the cave is located near the spectacular spoil wall at the EN side, about 10 m below the upper mine rim, in a slightly recessed area. The fissure in front of it becomes increasingly overgrown with bramble and young woody vegetation, making it harder to find during the vegetation period, as the entrance is not visible from below at all. In the essentially horizontally extending cave, a strong EN‑DNy orientation and an almost perpendicular tectonic direction are observable. The 72.5 m long cavity lies within a 16 × 18 m floor area, close to the surface. Its entrance is also its southernmost point. Its vertical extent is 10 m, almost the same downward and upward. From the entrance to the north, a few tens of metres away, an ancient karstic formation filled and covered with bauxite clay unfolds due to erosion. The cave is situated on parcel number 0198 of Csákvár.

Geological setting and development history

Hamvas Cave lies in one of the significant tectonic zones of the mountain’s structurally block‑segmented rock mass. The bauxite mine has very well exposed this special location, and during reclamation this part was left free. The broader geological structure is characterized by thick bauxite deposited on the karstified surface of the upper Triassic dolomite at the end of the Cretaceous period. After a terrestrial period, the area was flooded by the sea in the Eocene; it was covered with a clayey, coal‑bearing, marly, limestone series over the bauxite horizon. Along a huge EN‑DK‑oriented fault system (this is the visible mine wall) the EN side of the area was uplifted, while the DNy side sank. In fact, not only simple thrusting occurred, but also lateral and vertically twisting movements along the tectonic zone. All these were combined with thrusts, which can be seen on the rock protruding DK from the cave entrance.

The massive slip surfaces' shear zones (the displacement was about 50 m vertically) clearly illustrate the directions of movement. The movement did not occur in a thin plane but in a band at least several tens of meters wide, as a result of which the dolomite fragmented into pieces smaller than a centimeter, then formed a tectonic breccia that cemented into solid rock, while in several places the voids between the grains became calcified. The open cracks, a few millimetres wide and a few decimetres long, were also filled with calcite, and in the centimetre‑wide fissures calcite crystals sometimes grew.

The cave developed in this highly fragmented rock material at the intersection of several fracture lines (45–225°, 80–260°, 110–290°, 150–330°). Along the fractures, waters flowing in dissolved meter‑wide, mutually intersecting fissures into the dolomite, which is reflected in the cave’s characteristic branching plan. In the direction of impact (according to current knowledge) the fissures are not longer than a few tens of metres, and the vertical extent may be similar, although larger dimensions cannot be excluded downward. The phreatic dissolution could have occurred along some conduits, which in places widened into dome‑chambers by the merging of spherical vaults. All these processes could have taken place in a suppressed state, because the clay‑marble mixture deposited on the dolomite did not allow waters from depth to reach the surface directly, thus a closed cavity system formed from above.

Later, when a direct connection formed with the overlying sediments and the surface during the uplift of the area, the clay‑bauxitic sediment entered the cave in several meters thickness. It most likely filled most of the cavities, but later, by internal accumulation, it moved deeper. From the overlying rock (Eocene limestone?) water percolating and leaching lime (at the present entrance area) began intensive speleothem formation. The densely growing stalactite flags, draperies and flowstones over time became completely calcified, their sheet‑like structure reflecting rhythmic deposition. In the cavity lying in the tectonic zone natural collapses could have occurred earlier, but the massive collapse mass observed today is a consequence of the explosions accompanying mining. The cave itself was revealed during mining.

In 1993 the cave was first called Hamvas Cave in its literature.