The land moves very slowly but continuously. If we look back about 280 million years ago to the Permian, the last period of the Palaeozoic Era, the distribution of the seas and continents was very different from today. There was no Lake Iseo, no Alps, no Mediterranean Sea, and not five continents, but just one large continent called ‘Pangaea’, surrounded by an immense ocean called ‘Panthalassa’. The land corresponding to today’s Sebino was located near a vast mountain range called the Variscan Belt.
At the beginning of the Triassic, however, about 250 million years ago, the sea submerged the plain that had formed following erosion of the eastern mountains in the Variscan Belt, and an archipelago of large islands formed in its place. The water was shallow, warm, and very clear, ideal for the formation of atolls and extensive coral reefs, and the low tides exposed vast stretches of sand. Over time, these ancient environments gave rise to important rock formations such as the Main Dolomite and the Esino Limestone that make up some of the best known mountains of Lake Iseo, such as Corna Trentapassi and the summit of Monte Guglielmo.
At the end of the Triassic and in the Jurassic, between 200 and 150 million years ago, Pangaea began to break apart along extensive rifts outlining today’s continents. To the west of today’s Sebino area, a new ocean opened up: the Piemont-Liguria Ocean. The archipelago, coral reefs, and sandy expanses that had characterized the warm Triassic sea were submerged by hundreds of metres of water. The new Jurassic sea grew ever deeper.
Evidence of this is found, in particular, in an important Jurassic rock formation: the Domaro Limestone, which is commonly called the ‘Medolo Group’. Next to the peat bogs on Monte Cognolo, we can see a well-defined series of layers pertaining to this formation, where numerous ammonite fossils have been found. Studies of the ammonites have shown that the rock dates to 183 million years ago. In the past, the rock layers of the Medolo Group were used to build houses, churches, and monasteries, such as the monastery of San Pietro in Lamosa itself.
Between 100 million and 10 million years ago, from the Cretaceous to the Miocene, the African plate collided with the Eurasian plate in multiple successive phases. Thousands of metres of sediment lying on the seafloor, fragments of long-past geological eras, rose up and gradually emerged. The sea, which had occupied the Sebino area for more than 200 million years, began to recede, making way for a new mountain range: the Alps. The folds of the rocks and the faults, which can easily be seen in the mountains surrounding the lake, are evidence of this very important event.
In the last 2 million years, in the Quaternary Period, the climate at the latitudes here has changed several times, resulting in what are called ‘ice ages’. The greatest expansion of the ice that we know of occurred during the last ice age, around 18,000 years ago. At the time, the western slope of Monte Guglielmo was covered by more than 1200 metres of ice. About 12,000 years ago, the glacier that had long occupied Val Camonica and part of the plain slowly began to retreat. It deposited large rocks (glacial erratic) and an enormous amount of debris near the lake and in Franciacorta, leading to the formation of today’s moraine hills. Their analysis allows us to reconstruct the extent, thickness, and path of the ancient glaciers.
As the ice receded, the basin of Lake Iseo also formed, which also included today’s peat bogs. The lake had two outlets: the Oglio, near Sarnico, and the Longherone, which continued towards the plain. As the water level dropped, a moraine hill just a few metres high separated and divided the lake basin from the basin which would become the marshy area of the ‘lame’.