Trascrizione
In this room, 80 rock samples and over 200 mineral specimens are on display, together forming part of the collection donated by Giacomo Perego in 1974, which gave rise to the museum you see today. All of these specimens come from throughout the Valtellina and its side valleys and are arranged here based on when they were formed or transformed into what they are, from the most recent to the oldest. In Showcases 11, 15, 19, and 20, you can observe samples of igneous rocks, those that formed through the cooling and solidification of magma either outside the Earth's crust, known as effusive, or within it, known as intrusive.
The sedimentary rocks in Showcase 17 require aquatic environments where sediments can slowly accumulate over time and solidify into rock. These are marine seafloors that, uplifted during the orogenic process, now form the peaks of the mountains around us…
Lastly, the metamorphic rocks in Showcases 13 and 21 are those that, in the collision between the two plates, were subjected to such high pressure and heat that their chemical and physical composition was altered. By observing where this type of rock has been found in Valtellina, you can get an idea of where the point of greatest turmoil and upheaval occurred.
Please approach now the model that replicates a fossil that is unique in the world, both because it attests to the existence of Cassinisia orobica, a conifer now extinct that thrived in these valleys around 260 million years ago (which, using the person-Earth comparison, would correspond to two and a half years in Gea's life), and because the fossilisation process involved extraordinary circumstances!
If you move towards Showcase 18, you can admire the original fossil, discovered in the 1970s by an international group of paleobotanists, that is experts in fossil plants, in the nearby Valgerola. The accompanying photographic enlargements tell the incredible series of random events that led to its fossilisation.
Try to imagine: some branches of Cassinisia break and fall into a freshwater pool, sinking into the muddy bottom.
Shortly thereafter, the water recedes, and the remains resurface and dry out. Then, the water returns, and large colonies of blue-green algae, still typical of many lakes and oceans today, develop on the submerged plant. These algae have the ability to absorb the limestone present in water and grow in concentric layers, forming masses called stromatolites. In the photographic enlargement, they correspond to the grey-blackish parts… Over time, these colonies of algae grow around the fallen plant branches, and as the plant decomposes, the mud of the seabed fills the cavities left by the plant tissues. Through the passage of geological eras, through very long processes of compaction and cementation, the mud transforms into rock.
Et voilà, along with the effect of the stromatolites, this transformation now presents us with a fossil that is truly one of a kind in the world!