IMAGNA VALLEY

 

SUMMARY

Imagna Valley is situated in the province of Bergamo, in Lombardy (Italy). It's a very karst area: there are about 200 caves. The highest mountain is Mt. Resegone, that is 1875m high. The vertical caves are placed from 1400m to 850m, the horizontal from 900m to 400m. The longest caves is Complesso Tomba del Polacco-Ol Böc, 4000m long. Then there are Il Forgnone, 2800m; il Büs della Siberia, 1570 m; Alaska, m 1450; la Grotta dei Morti, 700m; il Büs del Ciarì, 600m. The deepest caves are Il Forgnone, +170 m; Alaska, -146,+9m; la Grotta dei Morti, +137m; il Büs de la Siberia, -82, +34m; il Complesso Tomba del Polacco-Ol Böc, +112m; la Nala di Sciupì, -92m; l'Abisso Vuotodaria, -86m; la Nala di Rote, -78m.

 

SHORT DESCRIPTION

Imagna Valley is one of the catchment basin of the ring in the foothills of the Alps and it is very near Po Valley and a big town: Bergamo. 50m near Milan, we could suppose that its naturalist, geologic and speleological aspects are well-known, but this isn't true because a lot of this aspect aren't well-known. The territory of Imagna Valley is very rich of karst phenomenon: it is one of Bergamo areas with the biggest number of caves. There are about 110 caves on the official cave register and 140others are well-known, in all 250, then there are a lot of them that none has found and explored.

The geologic formations, in which the karst caves of the valley open, belong to Trias and they are three main kinds: Dolomia (dolomite) Principale (Norico); Calcare di Zu (Rhaetian); Argilliti di Riva di Solto (Norico - Rhaetian) - (Bus del Cunì - Complesso Tomba del Polacco-Ol Böc etc.). Dolomia Principale is a light rock, it isn't stratified, and it is particularly compact and strong. Calcare di Zu is made of blackish, marly, stratified and less strong rocks. Argilliti di Riva di Solto are black and quite friable argillites. In Imagna Valley they include karst interested limestone and black marl. The outcrop of dolomite is easily identifiable with steep walls, dry cliffs and not rich soil, while the outcrop of dark limestone is marked by a slight slope and rich soil. The areas, in which there are the main karst phenomenon, are five: the first is Valpiana (Costa Valle Imagna and Bedulita) and Cà Contaglio (S.Omobono Terme), the second one Palio over Brumano (Calcare di Sedrina, Dolomia Principale), the third one Camozzere (Dolomia Principale), the fourth one Rota d'Imagna (Calcare di Zu and argilliti di Riva di Solto) and the fifth one Val Marcia (S.Omobono Terme) and Cà Todesk (Costa Valle Imagna). Both the caves that are present in these areas, and the other ones that opens in the fracture in the compact dolomite or in the black strata of limestone, are very interesting for the beauty and richness of concrections, for paleontological and archaology find and for the presence of rare species of cave fauna.

 

THE KARST IN IMAGNA VALLEY

Sinkholes are a karst example, some of them are very large, it is possible to see them at Cà Todesk, hamlet of Costa Valle Imagna, they are made by the crash of a cave, a lot of them are more than 10m wide and nearly ten are 30m wide. Other sinkholes are at Cà Gazzoli, hamlet of Costa Valle Imagna, one of them is 70m wide and 30/40m deep. An other small area with a lot of sinkholes is under Cà Bagazzino of Costa Valle Imagna: there are 12 sinkholes, 3 swallow hole and 2 caves 3 inghiottitoi di cui 2 grotte. At Piazzo (hamlet of S. Omobono Terme) it is possible to see some beautiful deep sinkholes in the grass. On Costa del Palio (Brumano), at 1400m a.s.l., there are some very interesting sinkholes. First of all because they are big (10m), then because of their location, in the middle of Val Tetto and Val Foppa (near one the one hand to Abisso Vuotodaria and on the other hand to Bus della Siberia), and finally because in Winter they melt ice. Near the refuge of Mt. Resegone at about 1700m there is a that leads to "Buco della Neve" (Snow hole), that is an enormous sinkhole where the snow resists till late season because the sun rarely lightes it. It lies at 1750m a.s.l., it is 15m deep from the point where we come, while about 50m deep from the top of the walls, it is 15m wide and 100m long. It is filled by stones and there are to little hole that ends after some metres. In Mt. Resegone there is an other beautiful area on the 1815m top: there are 7 sinkhole from 1750 and 1815 m a.s.l., two of them are 30m wide and 20m. Then there are a lot of them in the woods, where it is difficult to see them. There is the "block karst" and needle of rock at Bedulita, Costa Valle Imagna, in the area of Cà Gazzoli, Cà Todesk, Cà Bagazzino. It is a wonderful place: very deep and very long carsic fractures and lapiazs adorn the woods. Lapiazs are very common on the crest of Albenza, at Pralongone and at Cà Gazzoli, Val Muraca, Cà Todesk, Cà Bagazzino (Costa Valle Imagna).There is the phenomenon of "wind tube", a winter vent hole with a warm air flow, that melts ice: at Neerola, in the village of Locatello, in Mt. Tesoro, at Costa Valle Imagna. In the side of Carenno (1426m a.s.l.) one of this vent hole is almost opened. Because of karst there aren't rivers, expecially in the highest area of this valley, because the most part of water is swallowed. An other karst phenomenon is periodic spring, like O Sbadol and Ol Gass, at Valsecca and Grota dè Val d'Adda (irregular intermittent), at S. Omobono; in which there are some sumps that causes the intermittence.

 

THE SIDES OF THE VALLEY

We can split Imagna Valley in two hydrographic sides: the left one and the right one. The left side icludes Brumano, Fuipiano Valle Imagna, Costa del Palio, Berbenno, Rota d'Imagna, Locatello, Ubiale- Clanezzo. The right one includes Costa Valle Imagna, part of S.Omobono Terme, Roncola S.Bernardo, Bedulita, Valsecca, the massif of Albenza. Watershed is Mt. Resegone.

 

LEFT SIDE

This side is delimited by some high mountains in the North: Mt. Resegone (1875m), Zucco di Pralongone (1503 m), I Canti (1563 m), Palio, Zuc de Valmana (1546 m). In the South height of mountains slopes down till the mountains becomes hills. Argillite di Riva di Solto crops out at Ponte Giurino, Corna Imagna and Rota d'Imagna with metric beds that alternated considerable thicknesses of Quaternary covering. The caves are mostly horizontal, influenced by the way of fractures. There are a lot of big springs (e.g. Val Vanzarolo), very long and complex caves (Il Forgnone, Alaska, Complesso Tomba del Polacco-Ol Böc).The beds of Palio are calcareous and have variable inclination. In the extremities, composed on the one hand by Mt. Resegone and the plain of Brumano and on the other hand by the plain of Fuipiano Valle Imagna and I Canti, inclinations vary between 10° and 15°, while in Costa del Palio (central area) are between 5° and 10°. Stratigraphic series include (from the younger to the older): Calcare di Moltrasio, Calcare di Sedrina, Dolomia a Conchodon and Calcare di Zu. It's about the same for the area of Palio in the side of Taleggio Valley. Stratigraphic series present characteristic of continuity, but in some zones the succession lacks some (e.g. at Morterone there isn't Calcare di Sedrina). This side is very important because there are a lot of (with the same characteristic of that one in Imagna Valley), from which La Maddalena (Lo Co 2654, over than 10 km of lenght) and the spring Fiom Latt (Lo Co 1259) stand out; Fiom Latt could be the mouth of all this enormous drainage basin. Imagna side is rich of springs, too; but they are smaller than Fiom Latt, but there is series of collectings of which we don't know the flow. A difference between Imagna Valley and Taleggio Valley is that in the second one the base level is in the contact between Dolomia a Conchodon and Calcare di Zu, while the last one is karst in Imagna Valley (e.g. Il Forgnone). The main drainage basin of Palio is located from 950m and 1100 m a.s.l., while the main springs are at 900 m a.s.l.. At 800/850 m a.s.l. there a lot of collected springs that could represent the table level. Moving to South we reach Corna Marcia and then Mt. Ubione, that has a thick wood, with steep valley, in which rivers flow only in rainy period. So we understand that karst phenomenon is present here, too; even if this area hasn't been fully explored.

 

VERSANTE DESTRO

The Northern side is delimited by Mt. Resegone (1875 m), Corna Camozzera (1452 m), Ocone (1410 m), Mt. Tesoro (1432 m). Moving to South there is Mt. Linzone (1392 m), then the crest descends. There are various slotted and permeable calcareous and dolomite rock, so the rain is easily swallowed and superficial water is rather scarce. Lithologic units crop out, the continuity of stratigraphic succession is broken off by some faults that influence the disposition of springs. These one are aligned along preferential bands: for example in Val Ronco there is an alignment at 580-700 m a.s.l., from which there is a series of rivers that go Imagna river. At Costa Valle Imagna and Bedulita (900-1000 m), there is the drainage basin with notable diffusion of sinkholse, swallow holes, potholes, abysses (Nala di Sciupì, Nala di Rote) in Dolomia Principale that isn't completely pure, infact there are often quartz crystals and fluorspar (Bedulita). Under there are the springs with horizontal caves in the same kind of rock. They are mainly developed along South-West/ North-East fracture and interstrata that are perpendicular to these systems of joint. We can't exactly define the hypogeous system and the connection between a cave and the other ones, but we can suppose the presence of a suspended fault at 850 m in Southern areas and it goes down going to North. The structure of this side isn't simple because of interacting of tectonic phenomenons with the karst ones. The valley floor is made of a bed of rocky and gravelly detritus. A clear example Valsecca. Seeing it from the top we understand that is over a detrital cone that has gone down from Mt. Resegone and Camozzere. Going in the highest areas of Albenza, the caves and the surface karst phenomenons are less frequent than in the zone located from 900m to1000 m. This is caused by the rock change. Between the valley floor and the highest areas there is an intermediate place that is steep, rich of deep gorges.

 

 

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This page is written by Andrea Maconi (Gruppo Grotte Milano)

Last Update: 20-12-1999