Sentiero tematico geologico sul tracciato del Monte Cecilia

Geological Thematic Trail
Thematic trail in stages along the route of Monte Cecilia

•Departure: Baone
Monte Cecilia is a laccolith. Magma intruded along stratification planes (that is, between layers) in a concordant manner (without interrupting the layer's continuity) arching the roof of the sedimentary cover into a dome. Its shape is plano-convex and appears flattened at the stratification joint, the surface that limits an underlying and an overlying layer; often this latter is an erosion surface or a surface of non-deposition of sediments. The first rocks we encounter on our itinerary are magmatic rocks, in the case of Monte Cecilia we find latite, an effusive rock mainly composed of feldspar. Latitic products are the latest in the Euganean eruptive cycle, indicating that the composition of the magma becomes progressively less acidic.
Why do we see magmatic rocks before sedimentary rocks, if they lie beneath the sedimentary cover? Because exogenous agents, that is, all those agents that act by altering rocks, such as atmospheric agents, water, ice, etc. have eroded the sedimentary rocks. The eroded material is redeposited as a blanket of debris. Erosion, over time, shapes the entire landscape and by abrading the sedimentary surface from the dome of the laccolith, it reveals the eruptive rocks.
Latites. They are part of those rocks that are low in silica, but rich in calcium and sodium minerals (plagioclase) and iron and magnesium (micas, amphiboles, pyroxenes). They are rocks of effusive magmatic origin (from magma that solidifies and crystallizes on the surface) and exhibit a porphyritic structure (large isolated crystals in a glassy groundmass). The latite found along the trail shows onion-skin exfoliation (the rock appears to flake in layers like onions). This is a form produced by the alteration of rocks, in a timeframe much more recent than their emplacement.
One curiosity: they are not pillow lavas. They are not pillow lavas, although they resemble them! The reasons are these: pillows display glassy material at the edges, which is absent in onion-skin exfoliation products. The fractures in pillows are radial, while they are concentric in latites; the vesicular cavities are radial in pillows and chaotic in latites. Furthermore, pillows are products of basalt eruptions, that is, basic rocks erupted in oceanic floors, while latites are more acidic rocks typical of continental or shallow marine magmatism.
Pillow lava: a typical product of subaqueous basaltic eruptions. They are rounded structures that cool, creating a glassy crust. They have a peduncle at the part from which the emission starts, thanks to which the orientation of the pillows in the outcrops is established. They are typical, for example, in Hawaii.

Rocks along the path: Scaglia Rossa. They are fine-grained limestone rocks, containing levels of flint. This sediment has a marine origin, from open sea, with a paleontological content of planktonic microfossils. The study of planktonic foraminifera allows detailed subdivisions of the biostratigraphic units of the Upper Cretaceous, enabling accurate correlations between stratigraphic sequences geographically distant from each other. The Scaglia Rossa has a Late Cretaceous age and in the southern hills extends up to the lower Eocene (from about 100 to 55 million years ago).
Scaglia Rossa - lower member. It is well stratified and at the beginning of the trail, the stratification is decimetric (there are 10-20 cm between layers). The color is reddish due to the dispersion of iron oxides. In the photo: the Scaglia Rossa at centimeter levels, in the western area of Monte Cecilia.

Curiosity: Flint is a rocky material made up of poorly crystallized silica that often forms from the accumulation of skeletons or siliceous shells of marine organisms. Hardground A peculiarity of the Scaglia Rossa is the presence of hardgrounds: they are dark red or ocher crusts, centimeters thick, made of aluminum and iron oxides and hydroxides. These hardgrounds underscore the interruption of sedimentation, resulting in a "stratigraphic gap" (lack of sediment deposition), due to variations in the chemical and physical characteristics of the seabed.

Scaglia Rossa - upper member. The stratification is very dense, laminated, with a "scaly" appearance. The upper part of this formation is more marly (has a higher clay content) and there are also single clay interlayers; here, it is free of flint. In the images, aspects of the Scaglia Rossa along the path. The most scaly can be seen in some small valleys off the main trail.
For some stretches of the path, it is possible to observe this formation of yellow-white color rather than red-pink. Its possible whitish coloration may depend on the lack of oxides, but it appears to be secondarily discolored due to contact with the underlying magmatic mass, as a result of alteration processes.

Overview to the East. Sudden, detached, austere; almost a self-contained entity were it not for two protrusions that keep it connected to the hilly complex. Monte Ricco extends in a final farewell to the Rocca; it, small and distant from the acropolis, the last drifting island of the eastern plain.
Monte Ricco (329 m) is made up of alkaline rhyolite, a rhyolitic rock rich in sodium and potassium. In a few places, it is possible to observe how the overlying Eocene marls, clayey limestone rocks, gray-green, are discordantly cut by the rhyolite. At certain faults, subvertical, basaltic veins have been injected. Monte Ricco has a trilobate appearance, thanks to a spur to the west (M. Castello) and one to the north (Dosso Solone).
Colle della Rocca, near Monselice, is 151 m high and is composed of quartz-trachyte. In the quarry, trachyte shows a beautiful colonnade fracturing typical of cooling and clearly shows how it lifts and verticalizes the Cretaceous limestones. It seems to have formed a sort of tower; an obelisk of lava with an uncommon shape, even for viscous lavas.

Overview to the northeast. Monte Orbieso (330 m) is a magmatic body of rhyolite, intruded beneath the sedimentary cover and originates a narrow elongated ridge extending towards Valsanzibio. The Monte Ventolone-Piccolo group is a rhyolitic mass whose intrusion has uplifted and tilted the Scaglia Rossa outward, except on the northern side where it seems to have poured out in the form of a flow.

Overview to the north. At 601 meters, Monte Venda is the highest hill in the Euganean Hills. It is a stagnation mound, that is, an accumulation of rhyolitic lava near and above the emission fissure; since it is viscous, it did not immediately expand, but cooled around the area from which it emerged. All this above basaltic rocks and Eocene marls. The main fissure should be located in correspondence with the morphological axis of the hill.

Overview to the west. Monte Cero (409 m) rises alongside Monte Castello and the village of Calaone. It is a probable eruptive laccolith: the trachyte has lifted and then torn the cover of sedimentary rocks, fragments of which are preserved on the northern side, slightly metamorphosed due to high temperatures in contact with magma. Also in this case, the trachyte of Monte Cero has partially expanded in brief flows.

Overview to the south. The morphology of Monte Cecilia (199 m) observed from the Moschine-Sassonegro area confirms the structure of the laccolith, with the dome-like uplift of the sedimentary covers. At the summit of the hill, there are the perimeter walls of a medieval castle destroyed by Ezzelino during the wars against the Estensi; legend has it that one of the causes was the beautiful Cecilia who resided there and from whom the name of the hill derives.