Structures cylindriques verticales dans un dépôt meuble Quaternaire, en Galice (Espagne)

Abstract

This note reports for the first time the occurrence of vertical cylindrical structures in a Quaternary deposit, in Spain. The two structures were observed in a silty sand glacio-lacustrine deposit in a valley near the frontier between Galicia and Castille-León provinces, in the NW of the Iberic Peninsula. The main structure is at least 225 cm long and 15 cm at its base, 20-25 cm in the central section, and about 40 cm in the upper section. The lower section is essentially sandy and characterized by circular layers. The central section is made of two superposed cones of small sandy gravel, whereas the upper section, which is intruding through a coarse-grained deposit, is filled with medium size gravel. The two vertical cylindrical structures observed in Galicia most likely result from ascending springs related to hydrologic pressures in the glacio-lacustrine deposit caused by the overlying outwash deposit. The structures are probably contemporaneous of the outwash deposit.