The sedimentary rocks of the Romagna Apennines.
While to the north of the via Emilia the Romagna area is essentially flat, the hinterland, south of the via Emilia, is characterised by a gradual but rather continuous development of hilly reliefs, at first very gentle and rounded and then, as one proceeds south towards the Apennines ridge, more marked, with clearer and more prominent shapes. Thus, we start from the very low altitudes of the areas of the plains (a few tens of metres above the sea level) up to over a thousand metres of the mountain ranges of the Casentinesi Forests (Foreste Casentinesi).
The rocks that make up these mountain ranges are sedimentary rocks: rocks, that is, made up of sediments, or fragments, granules - of various sizes - derived from other more ancient rocks, eroded by atmospheric agents or by the action of meteoric and marine waters, which have been accumulated over the millennia and, over time, have "cemented" (a process called diagenesis) to form the rock formations that we can observe today.
For geologists, sedimentary rocks take various names, based on the size of the granules that constitute them, from coarse to fine, observable with the naked eye or with a magnifying glass or microscope. Thus, we have conglomerates, consisting of pebbles, gravel and crushed stone with granules from tens of centimetres up to 2 mm; sandstones, consisting mainly of cemented sands, with granules from 2 mm (coarse sand) to 1/16 of a millimetre (fine sands),siltstones or silts, with very small granules, from 1/16 of mm up to 1/256 of mm, and clays, formed by accumulations of microscopic granules, smaller than 1/256 of mm.
All these rocks are therefore formed by the deposition and accumulation of these sediments and this formation process occurs in the vast majority of cases by means of water, which first erodes the oldest rocks, crushes them, then transports them and finally makes them deposit. This is a phenomenon that has occurred on the surface of the Earth for millions and millions of years, and which continues to the very day, incessantly, from minor waterways to rivers and the sea.
The fact that sandstones, conglomerates or clays are deposited essentially depends on theenergy that the water had when it allowed to deposit those particular sediments. It is easy to understand how the largest and heaviest granules, the pebbles, can be transported only where the water has a great energy, therefore generally along the courses of the most impetuous rivers, at the foot of the mountains, or in coastal environments or sea cliffs, where the waves break with great force. Smaller and lighter sediments are transported farther forwards and settle where the water has less force: sands, for example, settle on the edges of medium-energy rivers, on shallow beaches or in the shallow seabed, where a certain force of the water is still present, which is given by the current or by the waves, but that is not so strong as to keep these grains in suspension. The clay granules, which are the smallest and lightest, are the last to settle, because even a very weak and light movement of the water is enough to bring them back into suspension: clays thus settle in very calm bottoms, away from currents and the movements of the waves, and therefore, generally, in the depths of the deep seas.
If we observe the rocks that make up the hillsides of central Romagna, we will discover that in the strip closest to the plains, they are mainly made up of clays, and the hills have gentle and soft shapes, given by the plasticity and easy erodibility of this rock, which as is well known, can easily soak up water and become plastic and tender. In this first band, however, due to the same ease of the clay in losing hardness, in being eroded and in collapsing, sometimes the gentle hillsides are interrupted by the characteristic phenomenon of the "ravines": barren areas, devoid of vegetation, characterised by unstable sharp ridges, tending to collapse, interspersed with a network of gullies within which the clay crumbles.
Moving towards the south, towards the Apennines ridge, instead, we find rocks that tend to be harder, with more massive and steep mountain ridges, mainly consisting of an alternating sequence of stratifications of sandstone and marlstone (a type of clay with a strong calcareous component), which is easily identifiable in the eroded rocky faces near the rivers. These stratified rocks are structured in "packs" of considerable thickness, which emerge for several hundreds of metres but continue underground for an overall thickness that in some points reaches two or three km. Given that the phenomenon of sediment deposition and consolidation is a phenomenon that occurs very slowly, we can understand how all this thick rock has been deposited over a very long period of time, of many millions of years.
This means that, for a very long period of time, in what is now the territory of central Romagna there was a sea of varying depth depending on the phases. This is not to say that the sea level was higher than its current altitude, but rather that the land that then made up the seabed was lower than today, and that, at some point, something happened that led the seabed to rise up to emerge from the sea and become proper land, a continental environment.