Description
It will certainly have happened to you, during your mountain excursions, to come across those curious forms of corrosion that sometimes line the surfaces of limestone: long parallel grooves that thickly cut into the walls of large blocks or, more often, the layers denuded by the erosion of ancient glaciers (laste). Scholars of geomorphology call them 'furrowed fields', or 'karst fields', and they represent one of the most conspicuous and beautiful forms of karstification, i.e. the ability of water to chemically dissolve limestone with the help of carbon dioxide. Once penetrated inside the mountain, the water will begin to flow into the fractures, widening them and starting the cycle of cave formation. Its shaping action, however, already begins on the surface, at the first contact with the rock, and the variety of forms it gives rise to has justified in recent years the publication by geographers of veritable atlases of these important aspects of karstification.
A few notions of chemistry
Water and carbon dioxide can combine in various ways (carbon dioxide, a gas, is said to dissolve in water) depending on the particular situations in which they come into contact.
On ground covered with vegetation, the disintegration of the latter causes the formation of a lot of carbon dioxide and the water has the opportunity to become more enriched with this gas, i.e. to become 'more acidic', more corrosive to limestone. At low temperatures, then, the water dissolves the carbon dioxide even more easily: and so in high mountains, where the rock is often devoid of turf, the very cold nival meltwater also possesses a strong corrosive power, increased by the fact that the snow can persist for many months until late spring and provide a continuous supply of water to the rock below. The result of this work of the water on the surface layers is often represented by a whole series of beautiful grooves, holes, indentations, basins, small crevasses, up to long rocky trenches or deep fissure shafts that are sometimes the gateway to larger underground environments.
The formation of these karst phenomena naturally depends on numerous other factors that can combine and contribute in various ways to the morphology of the furrows and basins: the structure, compactness, porosity and fracturing of the limestone, the slope of the rock strata, the presence or absence of concave areas or small bumps, and finally the type of precipitation itself, rainy or snowy, on bare rock or covered by soil. The nomenclature in use for these phenomena (mainly German) reflects the variety of forms of furrowed fields almost always linked to their different origins: grooves (Rillenkarren), small, shallow furrows, often parallel and separated by sharp ridges, linked to the action of flowing and dissolving rainwater threads; shower grooves (Rinnenkarren), straight or meandering, a few centimetres deep and up to several metres long rounded furrows (Rundkarren), generally wider and with blunt, rounded edges, formed under the ground cover (covered karst); crevasse furrows (Kluftkarren), wide and up to several decimetres deep, originating and influenced by more pronounced fractures in the surface. Finally, other manifestations of surface karstification are the dissolution holes (karst holes), tubular cavities of varying dimensions often aligned along small fractures, and the corrosion ponds (kamenitze, a Slavic term now commonly used in the international arena), cavities a few centimetres deep and often with flat bottoms that develop on the slightly inclined surfaces of the limestone.
Itineraries with important examples of furrowed fields in our region: Valle del Sarca-Valle dei Laghi-Monte Bondone
NAGO
We are travelling along the 'Maza' road that leads from Arco towards Nago, slightly uphill. 400 metres before the junction for Torbole, we notice on the left the light grey limestone layers descending towards the road. We leave the car at a lay-by (with a capital) and begin to climb up the rocky slope, paying attention to the small crevasses that open from time to time hidden by the rare vegetation. The Nago Lasta, which is nothing more than the surface of a rock layer laid bare by the slipping and crumbling of the overlying layers, presents an almost complete sampling of superficial karst forms: 'pen-like' grooves (2-3 cm deep and a few decimeters long), long shower grooves, both straight and meandering, karst grooves, karst holes and finally corrosion pools in the highest part.
CALODERS (ARC)
We return northwards for a few kilometres and at Arco we head into the Laghel basin. From the little church of S. Maria we ascend to the top of Calodri (the ridge north of the castle) where we find a vast furrowed field with some of the most beautiful karst morphologies in all of Trentino. The traces of the glacier's passage (erosion and smoothing) are evident in this area, on which karstification later completed the last phase of its action. The limestone patches are divided by a network of karst furrows, deeply carved by karst holes and lined by a great variety of grooves (Rillenkarren): comb-shaped (they run parallel on one side of a small watershed ridge), pen-shaped (they run on both sides of the ridge) or radial, around a large hole. On the way back, we extend our walk by a few minutes and go to photograph, just west of the "White House", a group of interesting basins on mounded rocks (smooth, rounded rocks, shaped into small bumps by the glacier).
TERLAGO
We set off again towards Trento, reluctantly neglecting other places with interesting examples of superficial karst (Pianaùra, in particular, on the slopes of M. Stivo just above S. Martino and Massone) as well as not entering the vast area of the Marocche, where almost every type of karst sculpture can be found on the large landslide boulders, which are now increasingly covered by vegetation. We are tempted not a little by a quick excursion to the laste in the vicinity of Lasino and its furrowed fields of the Pradel but finally opt for a reconnaissance of the karstification of Lake Terlago, intrigued by an old study that Giovan Battista Trener and his brother-in-law Cesare Battisti had completed back in 1898. We descend to Lillà just before Cadine and walk over the northern shore of the lake. The furrowed fields, especially fissure furrows and holes that reticulate the rock, are carved here in the reddish limestone of the Rosso Ammonitico, made even darker in several places by colonies of lichens that have invaded the surface. The beauty is completed by some corrosion ponds, a couple of which are quite large. Not far away, a few traces of the important Mesolithic site studied in the 1980s by the Tridentine Museum of Natural Sciences bring to mind scenes of prehistoric life on the shores of the lake, which still lapped the strata recently stripped bare by the glacier.
CASTELAR DE LA GROA
We are at an altitude of 800 m on the southern slope of Castelar de la Groa, 2 kilometres beyond Sopramonte, on the Bondone road. At the junction for Maso Camponcino, we walk along a small road that goes round the hump on the west side. After 250 m, straight on and to the right beyond a crossroads of forest roads, we come to a small valley of Castelar in front of a rocky wall (5-6 m) marked by some clearly visible shower furrows that line it vertically. To the right, a series of lighter, straight grooves testify to the flow of water threads along the maximum slope of the limestone. We go back up the wall on the left and at the summit, jumping from one rocky spur to another, we find ourselves in the middle of a real furrowed field: here too, dissolution holes and small karstic crevices divide the surface into blocks on which a vast sample of grooves is carved on all sides, with irregular directions or sometimes small diverging bundles. Many other places on the slopes of the Bondone bear evident signs of surface karstification: the surroundings of Malga Mezzavia, for example, along the road that goes up to the Viotte (grooves of nival origin); the Rosta ridge, above the Viotte and the Val d'Eva, with ample remains of rounded grooves now degrading due to the combined action of karstification and frost-thawing on a very fractured rock type; Finally, descending towards the Cavedine valley, the Lavachèl-Colmi area is worth an afternoon's excursion (a few furrowed fields, dolines in abundance, grottos, swallow holes and, towards the north, even a kind of small polje with an underground outlet that can be explored for about ten metres).
High mountain ranges: Brenta Group, M. Cornon (Latemar Group)
BRENTA DOLOMITES
On high mountain karst plateaus, corrosion occurs mainly under the snow cover and overlaps with the forms of glacial erosion that have 'prepared' and modelled the surface. Often, around an altitude of 2100-2500 m, the horizontal rock layers appear 'cut' and arranged in steps, and the forms of dissolution completely etch both the flat surfaces and the vertical heads of the steps (Schichttreppenkarst, structural stepped karst). Examples of this type can be admired in the Brentei-Alimonta area and especially at Grostedi, where the dolomite shelves (not pure limestone, therefore, here, but calcium and magnesium carbonate) are thickly incised by long fissure furrows that often completely cut the step, joining the underlying horizontal cavities that form between layer and layer. However, moving between the shelves of the Grostedi requires a certain amount of care, because in the midst of the smaller grooves, large crevasse shafts suddenly open up here and there, their bottoms blocked with snow and ice, some of which (recently explored) reach a depth of 80-90 m.
An hour's walk to the west, immediately below the Tuckett hut, we encounter instead a classic and more 'typical' example of a ploughed field with karst holes, a few basins and long shower furrows cut transversally by small karst-cracks.
Finally, in the southern sector of the Brenta, at the head of the Val d'Ambiez (Alpe Prato), there is an important area of 'mixed' furrowed fields, with shapes of both nival origin (Rinnenkarren) and covered karst, with more rounded contours (Rundkarren).
M. CORNON (Tesero)
A beautiful excursion, perhaps to be combined with the better known Doss Capèl-M. Agnello geological path. Agnello, not far away. From the Malga di Pampeago we ascend to the Caserina and head towards the Cornacci. Already in the vicinity of the Caserina we begin to find a few dolinas and a few small caves, which will become more numerous (about fifteen) along the entire rugged western slope of the Censi, below the ridge on which the trail runs. At the Baita della Bassa hut, on a short ledge overlooking the Dos dai Branchi, the erosion of the head of the Rio Bianco valley and the removal of the turf have partially laid bare a small rocky amphitheatre here and there incised by short grooves-crevasses (kluftkarren), a few grooves and a few corrosion pools (kamenitze).