Geological history of Poland...and the remains of an ancient, tropical sea that covered central Europe millions years ago
Millions years ago, during the Devonian geological period, central Europe was flooded by the waters of a shallow, tropical sea. Diverse groups of animals inhabited the ocean, which separated Europe and Northern America from a large southern continent, Gondwana, which spanned a distance of several thousand kilometers, to the southern pole.
Much time passed by and immense mountain ridges emerged from the sea waters. In the Carboniferous Period, central Europe became a maze of high mountains and valleys, active volcanoes and deep basins, in which vast swamp forests gave birth to huge coal deposits.
Millions of years later, the inevitable geological processes of weathering and erosion brought the history of the mountains to an end. Large basins, wastelands and deserts covered by sand dunes were flooded in Late Permian times by a warm sea. Under the conditions of hot and arid climate, considerable masses of water evaporated, which led to the formation of huge layers of salt. The Lower Silesian copper ores (one of the largest deposits of this type in the world) also formed during the transgression of the Permian sea.
Marine waters returned to the central Europe and Poland again several times. The deep Jurassic sea was inhabited by large ammonites. Some of them were active predators that hunted in packs, while others fed on marine plankton. The sea floor was covered by living corals and sponges. Their fossilised remains are abundant in the Jurassic rocks of southern Poland.
Geological history and fossils in the most emblematic monuments of Poznań
The sediments that have been deposited on the floor of the Devonian and Jurassic seas and in the Carboniferous and Permian mountains are among the most beatiful Polish building and decorative stones, widely used in facades, columns, pavements and steps of historic monuments and public buildings of most of the cities and towns of Poland. Each of these stones contains a diverse assemblage of fossils of the sea animals, which can be easily found and identified, even by a non-palaeontologist, within polished slabs of the Devonian, Permian and Jurassic limestones and conglomerates.
Those decorative stones can be therefore be employed to popularise a vast amount of geological scientific knowledge, in particular the basic principles of the palaeontology, Continental Drift, the history of the Earth and the geological history of Poland and central Europe.
In order to make these scientific facts more accessible and attractive to the public, a short geological guide that introduces the tourist to the most impressive and widely-recognised monuments of the city of Poznan, Poland, has been established. This guide includes 4 locations in well-known historic churches of Poznan, in which decorative stones can be admired and fossils can be easily identified.
Archcathedral Basilica of St. Peter and St. Paul (The Cathedral)
The chessboard stone floor of the naves is made of two distinct rock types. The dark Devonian limestone from Dębnik near Kraków (southern Poland) contains clearly visible, numerous white and pale grey skeletons of fossils sponges and corals. These animals thrived in shallow waters of the warm sea, over 375 million years ago.
The much lighter, yellow tiles of the chessboard stone floor of the Cathedral are made of the Jurassic limestones from Morawica near Kielce, south of the Holy Cross Mountains in central Poland. Beautiful cross-sections of the fossils of extinct ammonites, belemnites and sponges are common in tiles made of this stone, whose history begun in the warm and deep Jurassic sea that flooded most of central Europe over 160 million years ago.
St. Francis Seraphic’s Church
A well-maintained, chessboard stone floor of the St. Francis Seraphic’s Church is composed of the Jurassic Morawica limestone, the same as in the archcathedral, and of much darker, brownish tiles of the Devonian limestone from the vicinity of Bolechowice village near Kielce in the Holy Cross Mountains (central Poland). Large fossil skeletons of sponges, bivalves and corals are visible as light, grey to white, oval and meandering, finger-shaped spots within virtually all the tiles of this decorative stone. The Bolechowice limestone formed in shallow waters, at a depth of less than 15 metres, in the Devonian tropical sea, over 375 million years ago, at the same time as when the darker Dębnik limestone from southern Poland was also deposited.
The Parish Church of St. Stanislaus (The Collegiate Church)
A heavily worn stone floor of the porch, next to the entrance to the church, is made of the Devonian Bolechowice limestone. Most of the floor in the Parish Church of St. Stanislaus is comprised of two-coloured, chessboard-like, brown and grey tiles of the Ordovician limestone, imported several centuries ago from the Öland, the second largest island of Sweden, located in the Baltic Sea. The tiles are very uniform; however in some of them long, light, arrow-shaped contours of large shells of nautiloids (marine animals that belong to the group of cephalopods) appear. Extremely rare today, during the Ordovician geological period, over 450 million years ago, they were common (and dangerous) marine predators. They reached several metres in length, although the specimens visible in the Parish Church floor are smaller, up to 50 centimetres long. The shells of nautiloids are the oldest fossils that can be found in the decorative stones of the monuments of Poznań.
The Church of St. Anthony of Padua
The floor of the church, which is located in the vicinity of the newly rebuilt royal castle, is made up of several types of decorative stones, the same as in the Cathedral, the St. Francis Seraphic’s Church and in the Parish Church. The stone steps of the altar, however, are made of the highly decorative, diversely coloured conglomerates from the vicinity of Kielce in the Holy Cross Mountains, from the same area where the Devonian Bolechowice limestone is exposed and quarried. The conglomerate is a coarsely grained sedimentary rock, which was deposited in deep valleys of high mountains that emerged in central Europe during the Carboniferous geological period, about 330 million years ago, when the Devonian sea left the area.
Partially rounded fragments of the Devonian limestone are clearly visible in the conglomerate. Some of them contain fossil sponges. The limestones that were laid down earlier in the warm sea of the Devonian Period were later uplifted and formed high mountain ranges. During the following millions of years, the mountain chains were eroded, and the rock fragments torn out from the bedrock were transported down the slope and deposited, forming a beautiful, decorative conglomerate that we can admire in the church of St. Anthony of Padua.
A new exhibit at the Earth Sciences Museum in Poznań
A visit to the monuments of Poznań presents a unique opportunity to study not only the history of Poland and the city itself, but also the worlds of the past, which were destroyed millions of years ago, together with their astonishing inhabitants. This unbelieveable history can be studied in detail in an exhibition in the Earth Sciences Museum at the Adam Mickiewicz University in Poznań. Most representative rock samples, polished slabs of decorative stones, fossils and graphical reconstructions of ancient ecosystems that represent over 400 million years of the Earth’s history are shown in 13 showcases. Lectures and workshops that accompany the exhibition further facilitate the assimilation of geological knowledge.