Some observations on the structure and morphology of an ice deposit in the "Abisso sul margine dell'Alto Bregai" Cave (Grigna Settentrionale, Italian Alps)

The Moncodeno area is found on the northern slope of the Grigna Settentrionale massif (Lecco, Lombard Pre Alps). The existence of perennial ice deposits in these caves at altitudes of about 1800 to 2000 m a.s.l. has long been known, the first written description dating back to Leonardo da Vinci. The ice deposit we investigated is found between two vertical shafts in the cave "Abisso sul margine dell'Alto Bregai", at a depth of about 80 m below the entrance, where no snow can reach it because of the cave topography. The deposit shows a more than 15 m thick succession of clear ice strata with crystal size ranging from about one centimetre up to several centimetres. Locally, clayey interstrata up to several millimetres thick are present at certain stratigraphic depths. Air circulation in this seasonally snow-sealed tube à vent cave produced a large tunnel in the ice. The deposit internal structure is dominated by an ablational angular unconformity separating a lower tilted and faulted part from an upper, horizontally bedded one. Below the stratigraphic gap a refrozen englacial drainage tunnel can be observed. The present ice deposit morphology shows a drainage network organized around two bédières collecting both ice meltwater and dripping related to rainfalls and seasonal snow melting outside the cave. A small channel leading to the bediérè from a shallow, flat-bottomed water pan with overhanging walls at the lateral ice-rock contact, some dripping-related vertical holes and widespread minor drainage features due to preferential ablation along crystal boundaries are also present. An interesting, stalagmite-shaped relief of uncertain origin is described. By the analysis of superposition and intersection relationships among stratigraphic, structural and morphological features we outline a relative history of accumulation and ablation stages. Building on previous findings we discuss some implications of these observations with respect to the ice deposit’s age and to hypogean and local epigean climatic evolution, and we compare the situation outlined in the 1973 topographic survey to the present one.