Geology

Geologically speaking, the Monterano Reserve is part of the larger Tolfa-Sabatini region and incorporates many of its representative features. The calcareous sediments which form the "floor" of the local geological series (but do not outcrop in the Monterano area) are overlaid by the well known Tolfa flysch. This consists of superimposed strata of marls (rocks midway between limestone and clay), shales (clays transformed into rock), sandstones (sand transformed into rock) and limestones, deposited in the ancient ocean of Tethys during the Cretaceous and Paleogene periods (between 90 and 60 million years ago). These sediments were then "transported" far from the place of deposition, confirmed by the highly "disturbed" state of the strata (originally horizontal, now intensely folded and fractured). These rocks of marine origin are widespread in the north and west of the Reserve (the La Bandita, Monte Angiano and Monte Ciriano areas). The marine sediments made up of clays and sandy clays, with the frequent presence of isolated gypsum lenses and crystals in limited parts of the Reserve (Poggio li Cioccati area), date from the Pliocene and Pleistocene (between 5 million and 1 million years ago). In the eastern part of the Reserve, this marine terrain is covered by volcanic terrain deriving from the ancient Sabatini volcano system (Bracciano area).
This includes the so-called "banded peperino" which outcrops in the Mignone, Palombara and Bicione valleys (formed during eruptions about 700,000 years ago), the tufas ("Bracciano tufa" and "black cinder red tufa" in the Greppa dei Falchi area) and lava flows such as the one visible near Casale della Palombara. The geological diversity has produced a varied landscape… rolling hills with wide fluvial valleys and gentle slopes where there are sedimentary rocks; narrow valleys with vertical walls where the tufa and peperino come to the surface. In a number of areas, there are also old mines (sulphur and manganese) and mineral water springs.

.
SEDIMENTARY rocks
.
The volcanic and sedimentary landscape of the Tolfa area has been shaped by the Mignone river and its tributaries.
About 8 km of the middle section of the Mignone crosses the Monterano Reserve, drawing a well defined boundary between the part with sedimentary rocks on the right of the river (facing downstream) and the part with volcanic rocks on the left.
The sedimentary rocks were formed by erosion and disintegration of earlier rocks by the sea, watercourses, glaciers and the wind, followed by deposition and compaction of the debris, almost always after being transported by the same agents.
 
In this area, terrain of sedimentary origin is made up of "allochthonous units", in other words, entire rock formations hundreds of metres thick which originated far away in the bottom of ancient seas or along the coast and were "transported" for hundreds of kilometres by the forces within our planet responsible for moving entire continents, raising mountain chains and creating new seas.
The most important of these formations is the "Tolfa flysch" formation. This strange name of Swiss-German origin is given to rocks originating from massive undersea landslides occurring between the Cretaceous and Paleogene periods from 65 to 23 million years ago.  This formation contains rock sequences made up of variously coloured slates (mid-Cretaceous) deriving from schistose mudstone (in other words, containing numerous superimposed "sheets" of clay-derived rock) and much harder and more compact sandstone ("pietraforte") (Upper Cretaceous - Paleocene) These formed in a shallow temperate-warm sea not far from the coast and the river mouths which transported vast quantities of sediments, periodically giving rise to gigantic undersea landslides.
 
The majority of sedimentary outcrops in the Reserve (the La Bandita, Monte Angiano, Monte Ciriano areas) consist of argillaceous-calcareous flysch (Upper Cretaceous-Paleocene) made up of clays and marly limestone (rocks "contaminated" by a high clay content). Particularly interesting is the presence of ruin marble (pietra paesina), a highly decorative rock with numerous small fractures and geometrically shaped segments of different colours deriving from the presence of oxides.  There are also smaller areas of more recent argillaceous rocks formed on the bottom of the sea several million years ago (but remaining more or less in situ, without being transported great distances as was the case with older sea bottoms), when the only dry land in what would later become coastal Lazio was a series of small volcanic islands.  Today the Tolfa Mountains, Mount Calvario towering 545 m a.m.s.l. above the village of Canale Monterano and the Ceriti Mountains are a reminder of the islands which once dotted the ancient sea, at its most evocative during foggy autumn days when they emerge above the dense blanket of fog covering the valley floor.
 
The sedimentary rocks here are clay rich, making them impermeable. They block water filtration and the subsurface is therefore almost completely free of groundwater.
The hills of sedimentary origin are the most affected by erosion, particularly in areas without woodland cover or overgrazed.  Sediment transportation figures (in other words, the quantity of debris transported by the watercourses) confirm the high index of erosion, amounting to 682 tons/km²/year of terrain eroded and carried out to sea.
 
This type of rock and the effects of the intense erosion have created a landscape of gently rolling hills and open valleys.
It is therefore particularly important to conserve woods and areas of scrub (essential to protect the soil and renew the woodland) and manage grazing correctly, ensuring that the livestock density does not exceed the capacity of the land (the number of animals that particular area, as a food producer, can support in a given period without causing degradation of the plant cover and therefore erosion).
.
VOLCANIC rocks
.
Over much of the Sabatini-Tolfa area, the sedimentary rocks described above are overlaid by volcanic rocks dating from two different periods of eruption and creating a more dramatic landscape, completely different from the gentle sedimentary hills.  It is precisely these contrasts which give the Reserve its extraordinary beauty.
The first phase created dome-shaped volcanic hills formed from hard, compact lava produced during volcanic activity in the Tolfa-Ceriti-Manziana area in the Pliocene and Early Pleistocene (between 4 and 2 million years ago). These were once the small islands mentioned above.
 
Then about 700,000 years ago, after a long period of relative calm, there was a great reawakening of volcanic activity, with formation of the Vicano system to the north (Vico-Cimini area) and Sabatini system to the south (between Campagnano and Bracciano).
Unlike the Vicano volcano, a classic volcano with a main central cone, the Sabatini volcano consisted of numerous centres of eruption generally in lines following fractures in the Earth’s crust from which magma, water vapour and gas flowed.
 
Rocks such as the "banded peperino" outcropping in the Mignone, Palombara and Bicione valleys were created by terrifying eruptions of volcanic clouds or ignimbrite (from the Latin "rain of fire"), one of the most frightening and destructive natural phenomena, producing a fiery (more than 800°C) mixture of gas, water vapour, molten rock and red hot boulders flowing at a speed of as much as 250 km/hour (depending on the gradient of the slopes) and destroying everything in its power. When this ceased, it left behind a mantle of red hot ashes which gradually cooled and hardened.  Geologists have identified a similar phenomenon in the terrifying destruction of the city of St. Pierre in Martinique (French Caribbean) in 1902.
The majority of the rock outcrops on the left of the Mignone River (facing downstream) originated from lava flows which produced hard compact rocks such as "banded peperino" and "black cinder red tufa". The latter is highly porous, with large amounts of black pumice or softer seams of pozzolana.
 
The exceptionally high mechanical properties of lava have always made it a popular material to make objects particularly at risk from intense prolonged mechanical action,  such as the paving stones used in the great Roman roads or, more recently, the cobbles and keystones for fortifications such as the Orsini Castle at Bracciano.
As it is easy to work and has relatively high mechanical qualities, tufa has also been widely used since ancient times as a building material, sometimes excavating tombs or constructions in the live rock, as occurred in the Etruscan burial grounds.
 
In the Reserve, the volcanic rocks in the Mignone, Palombara and Bicione valleys have been radically altered by the sulphides and sulphates produced by the gas circulating in the subsurface.
In the most heavily fractured zones, there are surface springs and gaseous fumaroles of carbon dioxide and hydrogen sulphide at temperatures above the local average.  These give rise to characteristic "solfataras" or mudpots, bubbling hot springs of milky coloured water, accompanied by the formation of mineral encrustations on rocks and organic material and pools of ferruginous water with a characteristic deep reddish colour caused by the intense mineralisation (one example is the Fosso Rafanello pool, still used by locals today).  Particularly when in contact with the hot mineral-rich waters, the rocks are subjected to intense mineralisation and this explains the mineral wealth of the Monterano area, extensively exploited over the centuries, from the 18th century sulphur mines (the most important was the Fosso del Lupo (or Biscione) sulphur mine owned by the Altieri family which produced 250 tons of sulphur in 1860) to the manganese mines of the 1930s and uranium prospecting in the 1960s.
 
Volcanic rocks are moderately permeable and water is able to filter through to the subsurface where it forms abundant groundwater, extracted by wells, often producing mineral water.
The small tufa plateau on which the village of Monterano (see below) stands and the plateau on the opposite side of the Bicione valley are formed by the superimposition of various volcanic rocks (Mignone banded peperino and "black cinder red tufa" produced during different periods of eruption). They are all that remains of the ancient almost flat (tabular) mantle of volcanic rocks formed by the ash and ignimbrites at the end of the eruptions.
 
The hard volcanic mantle, carved away for millions of years by the watercourses to form narrow steep-sided gorges (one of the most important ecosystems in the Reserve and the area as a whole), is interrupted by fissures which cut through the mass of rock in all directions and is crumbling as the rock walls gradually split into variously sized boulders.
Known locally as "castles" ("castelline") for their resemblance to fortified villages, these landscapes are among the most transient in Lazio and are doomed to disappear within a very short space of time geologically speaking.  A boulder rolls downhill, reminding us of the constant evolution of nature and gradual transformation of this, our Earth.
.
SOIL

.
Even we writers of guides for the public too often forget that the true miracle of life on our planet, at least life on land, is contained in the first 10/20 cm of soil, more commonly known as "loam". Soil may vary totally from continent to continent, from region to region or from one place to another perhaps just a few metres away, but it can basically be defined as a completely blended mixture of minerals (rock fragments, clay particles, water, air and other gases) and organic material including humus and millions of living organisms such as bacteria, algae, fungi, arthropods and earthworms, together with the remains of dead animals and plants.  The composition and thickness of the soil depend on the climate, type of rock present, quantity of water in the environment, gradient of the slopes, vegetation and numerous other factors.  Soil is life, as is proved by the existence of its opposite, the desert.  Soil in good condition and with good plant cover (from mosses to herbaceous plants, shrubs and trees) acts as a sponge, retaining the rain and gradually releasing it to the subsurface, protecting from flooding and supplying groundwater, wells and springs.  Protecting the forests means protecting the soil and the water and these are priority tasks for a protected area.
 
In the part of the Reserve with volcanic rocks, the soils are usually quite deep and contain 30-40% clay. The soil in the bottom of the gorges has the highest organic material and mineral content.  The higher the quantity of clay, the less rainwater is able to pass through the soil to reach the groundwater at depth. Volcanic soils are quite efficient at retaining rainwater, as shown by the abundant groundwater and springs in volcanic areas.
 
In parts of the Reserve with sedimentary rocks, the soils are usually thinner than in the volcanic areas (thicker where there is woodland, thinner where the pastures are overgrazed). They are richer in clay and rock fragments and have a lower rainwater retention capacity.

 

Ultimo aggiornamento: 3.09.2009 (15:30)   Stampa Stampa