GEOLOGY OF THE EUGANEI HILLS

Bike Touring Padova, Euganei Hills

The Euganean region is a striking and unique example of how geological events, the evolution of the natural environment, and centuries of human habitation, are so intertwined that they cannot be studied and understood separately from each other.

As an authentic open-air laboratory for the teaching of geology, the Euganean Hills emerge from the plains almost as a prologue to the not too distant Berici foothills and the pre-alpine slopes of the Alps; together, they share the history of an ancient sea that covered the area in the Mesozoic Era.

The oldest outcrops of rock are sedimentary and can only be found at Cinto Euganeo and in the Fontanafredda area. These rocks area identified as a rare type of red ammonite, a limestone dating back to the Jurassic Era (about 150 million years ago) that takes its name from the ammonites, extinct sea molluscs with a spiral shell.

Above the red ammonite, following a reverse timeline, we find other sediment layers.

Biancone is a clear fine-grained limestone, widespread in the area of Villa Teolo, near Bastia and Fontanafredda. Very different in aspect, but related, is a softer and darker rock, argillite, was discovered at the Bomba quarry near Cinto Euganeo, also found were important fossils from ancient tropical marine environments. You can see these displayed at the Museum of the Province of Padua at Cava Bomba, located the Industrial Archaeology Complex, a formerly an abandoned lime-kiln.

After the Biancone follows the Scaglia Rossa, an argillaceous thickly layered pinkish limestone that was formed by sea deposits between the upper Cretaceous and the lower Eocene periods, thus past the boundary between the Mesozoic and the Cenozoic or Tertiary eras. Commonly called "scale" (scaglia), it is certainly the most popular among the Euganean sedimentary rocks, especially in the area between Cinto Euganeo, Valle San Giorgio, Baone and Arquà Petrarca.  From the scaglia quarries, now no longer in use, have been found a number of fossil species, including sea urchins and shark teeth.

Chert, a gray, black or rose coloured rock of marine origin, is widespread among sedimentary rocks, particularly in biancone and scaglia. It was used by the ancient inhabitants of these hills, who skilfully exploited its properties to create sharp tools for hunting and daily use.

In this hilly region, the sedimentary series ends with the Marna Euganea. A clay-rich material, the marl is very brittle, tender, stratified, and mostly gray, and still appears in outcrops dating as far back as the Lower Oligocene era.  The extraction of marl in the area of Monselice, Rivadolmo, and Cinto Euganeo, served in the past for the production of cement.

The Tertiary era saw the beginning of the great transformations due to the characteristic volcanic phenomena that still clearly distinguish the unmistakable lines of the Euganean landscape. Volcanism is clearly divided into two distinct phases.  The first activity dates back to the Upper Eocene (about 43 million years ago), almost simultaneous with other important phenomena in the pre-alpine region that contributed to the formation of the Alpine chain.

Submarine basaltic lava eruptions, very smooth and rich in gas and vapour, poured and expanded unto the bottom plate of the ancient sea, forming low and fast solidifying accumulations upon contact with the water, and large amounts of exploded products, such as ashes and lapilli, formed thick layers of tuff among the mud of the seabed. Today, what remains of this first phase is found in the volcanic outcrops of the Central Hills.

It was only the second volcanic phase that gave its final shape to the present region. After a period of quiet, the early Oligocene (about 35 million years ago) saw a strong resumption of volcanic activity. Acidic and very viscous lava materials, rich in silica and of very different composition than those of the first phase, flowed abundantly to create a unique area in the Venice region.

From the cooling, arose rocks like trachyte, rhyolite, and latite, and strands of basalt. The strong push of magmas lifted and fractured in various ways the ancient layers of the seabed that until then had kept its original shape. This is when the new morphology of the Euganean region, with its unique and charming character, was formed. The gently rolling and rounded hills, called Laccoliths, consist of a swelling of lava which cooled underneath a roof of older sedimentary rocks. Their name comes from the erupting laccoliths that broke through the limestone roof when the magma continued its upward momentum. There are many steep and rugged features that were formed by the release of magma, like the spectacular trachytic rock wall of Rocca Pendice, between Castelnuovo and Teolo, best known today for its numerous climbing trails and for being a protected natural nesting site of the peregrine falcon.

At the end of the volcanic period, the highest peaks probably emerged like an archipelago of steeply rocky islets in the ancient Padano Sea. Much later, the emergence from the sea and the selective erosion of a million years, created a more varied landscape. Now the softer sedimentary cover has been eroded by the atmospheric elements. The evolution of vegetation and human settlement subsequently completed the work of transformation with major and decisive changes.

The varied geology of the land derived from the surface decay of the sedimental or volcanic top soils. The present morphology of small peaks with closed perimeters and variable exposure to the characteristic weather, make the Euganean Hills a very valuable region for biodiversity. The rainfall and the average annual temperature are similar to that of the surrounding plain, but, in the hills, the climate is milder and, in winter, the thermometer does not drop below zero as frequently nor for as long. This explains how the southerly exposed hills, bathed in more direct sunlight, can easily support vegetations of olive trees, cypresses, laurels, brooms and other unique species of Mediterranean flora. The cold season in the hills is often accompanied by dry days and clear skies, while the plain below seems to rest in fog. This phenomenon, due to thermal inversion, is very suggestive, bringing up the tops of the hills like islands in a sea of clouds, as a reminder of the expanse of water of the ancient sea that once characterized the landscape.

Similarly, in the summer, the climate of the Hills is cooler and less humid than the lowlands. In general, the greatest warming occurs on the southern slopes, while the opposite is true for north-facing slopes or within the narrow valleys.  On opposite sides of the same hill can therefore occur environmental conditions that create very contrasting microclimates. Important climatic differences not only depend on morphology and exposure, but on the inclination of the slopes and the presence of bare rock that reflects the warm sunrays, as in the many disused quarries.

Mining activities have left steep, calcareous, or argillaceous walls of sedimentary rocks, once used to produce lime and cement . Trachyte and other important raw materials of volcanic origin, were historically mined for the production of slabs and blocks to pave the streets and squares of the most noble parts of Padua, Venice and many other cities in Veneto. Sometimes, the wounds inflicted by man to these hills have been fatal, irreversibly transforming them or even erasing them off the maps. However, due to the slow but inexorable process of natural regeneration, and at times with the assistance given by man with specific projects, in some places the vegetation has reclaimed possession and created particular environments, new to the Colli Euganei.

Pioneer species of plants and animals now colonize the old rock quarries and contribute to an enriching biodiversity. An important natural resource of these hills, whose reputation crosses national boundaries, is without doubt the presence of one of the most important and extensive network of thermal springs and spas in all of Europe, centred on the towns of Abano Terme, Montegrotto Terme, Battaglia Terme and Terme Galzignano.

The nature of these thermal waters has been the subject of a study in the mid 1970s which retraced their origin to a system of underground geothermal circuits, therefore disproving the proposed theory of the existence of magna still latent deep underneath. The volcanism that created the Hills is now too old and exhausted to still generate an active source of heat. According to the proposed model, the waters originate from rain precipitation in the mountainous territory of the Little Dolomites (Recoaro, Valdagno, M. Pasubio) where they begin a slow descent along a system of fractures in the sedimentary rocks, reaching a depth of 3000 metres until they are stopped by an ancient impermeable crystalline stratum.

During their underground journey, which is aided by the geology of the Padana area and the pre-Alpine valley, they are constantly increasing in temperature because of the increasing heat deep inside the earth.

In the Euganean region, the resurfacing of the waters is made possible by the physical obstruction of the solid and compact volcanic rock and by a series of fractures that practically constitute a hydraulic pressure system. Enriched with a slight radioactivity and loaded with various components and precious minerals found in the rocks they filter through, they reach the surface. Their salinity is linked to their temperature: the warmer and deeper wells, which may exceed 87°C, have dissolved minerals of the order of 5-6 g/I. The most common chemical elements found are sodium, potassium, chloride, magnesium, sulphur, bromine, iodine and silicon.

The thermal waters are essential for the preparation, in special tanks, of the vegetal-mineral mud, which is the characteristic healing element of the thermal basin and is obtained from the spontaneous mineralization of particular microscopic algae. In collaboration with the Park Authority, most thermal spas have applied for, and some (Abano Terme and Galzignano Terme) have already obtained, the EMAS certification for Spas attesting their success in improving the quality of the environment and of the tourist reception

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