One of the centers most affected by COVID in this period was Bergamo, a city that, despite having gone through dark moments of misfortune and pain, has managed to preserve its beauty that resides above all in the imposing Venetian walls, a symbol of the province and famous throughout Europe, as well as a UNESCO World Heritage Site since 2017. But, what materials were used in their construction? Where do they come from?

The Venetian Walls, one of the most representative symbols of Bergamo, are an imposing architectural construction dating back to the 11th century, which extends for about 6 km along the entire perimeter of the Upper Town. This enclosure, still in good condition, consists of 14 bulwarks, 2 floors, two powder cases and 4 gates (San Giacomo, Sant’Agostino, San Lorenzo and Sant’Alessandro).

The main materials used for the construction of the walls are quarried from the site: gray sandstone (from the Castagneta area), yellow sandstone (from the Rocca area and from the ramparts of S. Michele and della Fara), conglomerates, white pink marble from Zandobbio and Sarnico sandstone. For the construction, ashlar blocks in sandstone, mortar from a mixture of lime and sand, bricks and timber are also used. Further materials are then obtained from the demolished buildings along the circuit of the Walls.

The wall face is mainly made up of blocks of sandstone or arenite, a clastic rock, of sedimentary origin, which is formed by cementing sand in different periods and layers. The granules can have various mineralogical compositions: the main detrital constituents of the sandstones are quartz, due to its resistance to abrasion and chemical alteration, feldspar, muscovite, biotite, heavy minerals. The mineralogical composition of a sandstone can give indications, even if only summary, on the rock’s origin. For example, an abundance of quartz and alkaline feldspar means that the sand derives from the disintegration of acidic or metamorphic igneous rocks; the sandstones of this group are generally mono-mineralogical and monocrystalline, ie with the granules of a single mineralogical species and consisting of a single crystal. Poly-mineralogical and polycrystalline sandstones derive from the diagenesis of sands formed by meteoric weathering, degradation and associated deposition of sedimentary, metamorphic schistose and acid eruptive rocks. In general, the sandstone has fair technical characteristics subject to alterations, a fair degree of compactness and resistance to mechanical stress and impact and high resistance to slipping.

In addition to sandstone, another historical stone material from the province of Bergamo, well known since ancient times and used in particular for the construction of the doors leading to the upper part of the city, is Zandobbio marble. In reality it is not a marble in the strict sense, but a compact crystalline dolomite, known in literature as the Dolomia di Zandobbio. From the petrographic point of view, the rock has an average texture with a saccharoid aspect and is made up of rhombohedral crystals of dolomite of diagenetic origin, with subordinate late spathic calcite and traces of native albite. From a physical-mechanical point of view, however, it is characterized by high compressive and flexural strength, as well as high durability. It also shows a good attitude to workability and a good degree of abrasion resistance. For these reasons this material has been widely used for the construction of some symbolic monuments of Bergamo, such as Porta San Giacomo. The deposit from which it was extracted dates back to the Jurassic era (about 200 Ma) and crops out in the foothills east of Bergamo, in the lower Val Cavallina and in the municipalities of Trescore Balneario and Zandobbio.

Among the stones best known and used since ancient times throughout the province of Bergamo and used in the construction of the walls, in particular for the redondone, there is also the sandstone of Sarnico. Abundantly widespread along the pre-Alpine belt, it, of turbidite origin and of the Cretaceous age, crops out along the connecting strip between the Prealps and the Po Valley. The finding of sandstone over the centuries has represented a widespread activity in the province of Bergamo, especially in the Sebina area and in Sarnico, where there is plenty of evidence of a considerable mining activity, now completely abandoned. The possibility of finding powerful courses in outcrop and its good physical-mechanical resistance are the basis of its wide diffusion. The Sarnico sandstone is suitable for the construction of masonry works and cladding in squared slabs; however, it fears stagnation of water and freezing and is subject to wear, which can also be manifested with phenomena of disintegration and surface pulverization. The stone, being very soft, lends itself to any type of processing and its mineralogical homogeneity combined with its technical properties allow for excellent workability.

Finally, conglomerates from the Roccolino mountain and from San Vigilio were also used in the construction of the imposing masonry. They are clastic sediments resulting from the dismantling of older formations by erosion agents or exogenous agents (meteoric agents, currents, landslides …). A conglomerate can be polygenic when it is made up of clasts of different types, or polymytic when it is made up of clasts of different sizes. In geological literature, conglomerates are traditionally divided into breccias and puddinghe. The former are rudites, whose sediment is made up of gravel, characterized by sharp-edged pebbles and low textural maturity as the granules are poorly classified and have different sizes. This could be due to a short “transport” which did not allow a good classification and a good rounding. Puddinghe, on the other hand, are conglomerates in which the pebbles, or clasts, are particularly rounded and have greater “textural maturity”, indicating a longer transport.


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