Mercoledì Nov 22

Geology in the Bergamo area

This room provides an account of the major geological events impacting the Bergamo area over hundreds of millions of years. We have here an imaginary journey through time, from the Paleozoic era to the present day. This geological history is presented chronologically, and each cabinet refers to a single period, with descriptions of the main geological events. At the entrance to the room there is a round table dating back to 1901. It was donated to the Museum by the sculptor, from Val Cavallina. The top is in Carrara marble. Inserted into the table are water-smoothed pebbles from the River Cherio. The table top includes a large group of kalinite crystals, an artificial sample obtained from a supersaturated solution. The large model in the middle of the room highlights the main geological structures of the province, classed by age and genesis.

The introductory cabinet outlines the characteristics of the area of Bergamo, inserted into a structural scheme for the central Alps . The oldest local stones date back to more than 360 million years ago. Pronounced metamorphism has deleted most fossils and original structures. The rock formations of this distant period mainly consist in gneiss, micha schists and phyllades, plus rare intrusions of eruptive rocks, with all outcrops in the northern portion of the Orobian Alps, in the vicinity of Valtellina. By the end of the Carboniferous period, the territory of the Orobian Alps had emerged, and consisted in a level plain. About 275 million years ago, the central and southern portion of the Bergamo area was still emersed land. It was the scene of intense volcanic activity. The northern portion was under water. Fossil findings in the stone of this period are very rare. Volcanic activity ceased toward the end of the Permian period, and the sediments transported by the rivers filled the old basin. The area, which had emerged once more, was level. The rock formations bearing witness to this geological period are mainly volcanic, sandstones and conglomerates. We see them today in the northernmost portion of the Bergamo area. We find exhibited here rocks with the fossilized footprints of amphibians and reptiles left on mud. One sample bears the marks of raindrops on an ancient beach of the Bergamo area. Some of the traces are relief, since they were preserved on the lower surface of the rock which covered the muddy plain upon which the footprints were originally impressed.

The large 1:25,000-scale relief model provides a faithful but simplified three-dimensional reconstruction of the geological map of the province of Bergamo. Colour coding is used for the diverse rock outcrops of the various geological periods in the area of the Orobian Alps. Each colour refers to a group of rocky formations which are of the same age. The oldest rock outcrops, dating back to the Paleozoic era, are located in the northern portion of the area. In this belt, metamorphic rocks (pink), argillites and sandstones (red), and conglomerates (green) prevail. The central portion of the Orobian Alps, making up the main portion of these mountains, is characterised by Triassic rocks. These are mainly limestones (grey and lilac) and dolostones (purple). In the southern portion, the belt of Jurassic (blue) and Cretaceous (pale green) rocks emerges. The Jurassic rocks are mainly limestones, and limestones with flint. The Cretaceous rocks are marls, limestones and argillites. Outcrops on the plain of rocks of the Cenozoic era (yellow) are sporadic, and consist in gravels and sands. The Neozoic era is represented by alluvial deposits (grey) totally covering the plains.

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The Mesozoic era includes three periods: Triassic, Jurassic and Cretaceous. The Triassic period, of between approximately 250 and 200 million years ago, is characterised by alternating cycles of warm shallow waters, in which large coral reefs formed, and periods in which the reefs disappeared. Toward the close of this period, the Bergamo area was covered by a warm tropical sea with extensive reefs. Triassic rocks now make up the main, central portion of the Orobian Alps. They are mainly limestones and dolostones, recording the presence of ancient reefs, marls, sandstones and blackish argillites. These rocks occasionally yield very large amounts of perfectly preserved fossils which are of inestimable scientific value. The Triassic rocks also host mineral deposits which once represented an important economic resource for local communities. From the Jurassic period on, about 199 million years ago, the Bergamo area began to sink. Deep basins formed, in which sedimentation of calcareous muds and flints took place. At the end of this period, the seabed was flattish and the water fairly deep. Many fossils of extinct cephalopod molluscs, ammonites, have left a record of the most widespread life forms of these ancient seas Jurassic rock outcrops are to be found, today, on the peaks of the hills bordering the plain. At the start of the Cretaceous period, conditions were as during the Jurassic period. Light coloured limestones were deposited up to about 100 million years ago, when the Alps began to rise up. Watercourses transported large quantities of material to the sea still covering the southern Orobian Alps. Massive turbiditic deposits, and also argillites, marls and sandstones, formed. On the surface of certain rocks we find a curious geological phenomenon – mud cracks – where we see typical polygonal structures forming as the original seaside sediments dried out. Cretaceous rocks now constitute the southern belt of the Orobian Alps, including the hills of the city of Bergamo.

The last cabinets in this room are dedicated to the Cenozoic period and to the local geological history of most recent times, the Neozoic period. At the start of the Cenozoic period, some 65 million years ago, light-coloured calcareous mud deposits formed on the bed of the sea covering the southern Orobian Alps. The pressures responsible for the rise of the Alpine chain raised up all the lands of Bergamo from the sea. About 5 million years ago, the plains were once more submerged by the sea, and this situation, recorded in lithological changes, persisted until the start of the Quaternary period, some 2.5 million years ago. The rocks are mainly light-coloured limestones, clays and sandstones. We also note diorites and porphyrites, associated with the cooling of magmatic intrusions. Nowadays, the Cenozoic era outcrops are, for the most part, covered by the alluvial deposits of the plains, and can only be found along the cuts made by the main rivers or in lithological samples collected by drilling. From the Neozoic period on, also termed the Quaternary period, about 2 million years ago, the sea retreated from the plains to the present Adriatic coastline. Fluvial sediments cover the plain land with a thick gravel coat. The rocks of the Quaternary period are mainly lake argillites and fluvial conglomerates. Marshland and ponds developing in certain valleys formed fossil-rich sediments. This was the era of glaciation, when deep grooves were cut into the hill country. These grooves are now occupied by the Lecco, Endine and Iseo lakes. The ancient lake basin at Leffe yielded the fossil displayed here, i.e. the mandible with tusks of a young southern elephant. The cabinet also contains fossils of ancient bisons and rhinoceroses – extraordinary animals which we nowadays associate with other continents, but which once lived here. Also displayed are cave deposits, with stalactitic concretions, and the fossilized remains of the cave bear and Homo sapiens sapiens discovered in the Bergamo area.

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