The Vis archipelago comprises the Vis island and the smaller nearby islands – Biševo, Sveti Andrija, Brusnik, Jabuka and Palagruža.
From geological perspective, it is the most attractive area of the Adriatic Sea encompassing both the oldest and the newest rock formations. Some parts of the Vis archipelago are built from volcanic rocks. Such rocks are unique in the Adriatic Sea and easy to differentiate from other Adriatic Sea islands that are predominantly of sedimentary origin.
Since ancient times the seafarers knew this area was specific. During the navigation in the vicinity of volcanic islands Jabuka and Brusnik, the compass needle deviated from the north, posing a potential danger at open seas.
In the Mesozoic, in the end of the Age of Dinosaurs, i.e. at the beginning of the Cenozoic or the Age of Mammals, Biševo was located in a tropical belt in the middle of the ancient Tethys Ocean. It was a part of a small continent located between Europe and Africa called the Adriatic Carbonate Platform. The small continent was covered by shallow, warm sea teeming with life and numerous organisms. A group of bivalve molluscs called the rudists lived burrowed in the muddy lagoons of that tropical sea. Nevertheless, an asteroid impact some 66 million years ago sealed the fate of these protozoa, together with the dinosaurs. Biševo’s sedimentary layers were built from numerous shells and remains of these organisms.
These sedimentary layers rose after the later collision of the Adriatic Carbonate Platform and Europe, creating the Dinarides and the present-day islands of the Vis archipelago.
The fine sand was formed of the millions of tiny calcareous shells of protozoa (foraminifera), algae, sea urchins, shells and their debris. The Quaternary period, when the sea level was 140 meters lower than today, was marked by dry and cold Ice Age. Strong winds carried the sand from the steppes to the surrounding hills and present-day islands. In that manner the fertile soil which yields the finest quality grape and the beautiful sandy beaches of Biševo were created. In the period when the sea level was rising, the sea currents moved the sand, creating sandbars. The accumulated grains of sand in the sediment have been subsequently bound by the crystallised carbonate cement, forming solid rock – the limestone. Thus, the newest pages in the stone “book” of the Biševo island are the limestones formed during the Paleogene, the geological period dominated by mammals.
It is easiest to read the stone “book” of Biševo, the consequences of the violent dynamics of the Earth’s crust, in the hinterland of the Blue Cave. Due to the action of the Earth’s tectonic forces and the slow movement of large blocks of the upper parts of the crust, the rocks were fractured, resulting in the blocks moving along these fractures – flat surfaces known as the faults. Traces of tectonic movement or scraping of the Blue Cave’s block against the central mass of island Biševo rocks can be seen along the sleek fault plane called the paraclasis. This fault intersects the rocks so it can be traced from the southern entrance to the Blue Cave to the northern narrow passage between the shore and the small islet along which the boats with the Blue Cave visitors are sailing.
The Blue Cave itself was probably formed through selective erosion (abrasion) during the interaction of the tectonic (diapiric) rise of the island and the rises of the sea level after the last Ice Age. More precisely, the rocks of the terrestrial phase are softer than the surrounding carbonate rock, and that very horizon, i.e. a “page” from the stone “book” is presumably located in the level of the submerged parts of the Blue Cave. Massive waves propelled by strong southern winds crashed into the softer rocks, carving out a large hole in them – the present-day geomorphological phenomenon, the Blue Cave. These rocks are still under the influence of the strong winds and waves that carve them.
People have been exploiting rocks since prehistoric times. There are various types of limestone on Biševo island which occur naturally in various forms – as massive, thickly bedded, nodular (wavy) and platy. That natural material was thus used for the construction of houses and other objects on the island. On Biševo flatlands there is a spacious plain (Polje) with the island’s largest settlement bearing the same name. The Church of St. Silvester is the most prominent building on the island, built from various types of meticulously picked Biševo stone (carved blocks, boulders and plates). The second important building on the island, former school and present-day Visitor Centre Modra špilja – Biševo, is also made of the autochthonous Biševo stone.
Besides construction, the inhabitants of the island used various types of geological substrates for various crops that supported life on this distant island. Due to the specific geological substrate, the highest quality of the soil is in the Polje area, offering breath-taking views of the Svetac island. The area is gorgeous in spring, when macchia turns into a floral oasis.