The Grotto of Casteret is formed in Cretaceous limestone.[8] The ice in the Grande Salle is ponded and has formed as the result of cold air flowing through the system, with areas out of the draught being water.[9] The deeper ice is considered to be very old, and possibly relict of a cold period of the Quaternary.[3] The volume of ice in the Grande Salle has decreased since its discovery. In 1950 the entry into Salle Maude was a low crawl across the ice floor about 30 centimetres (12 in) high. In 1988 it was reported that the gap was 150 centimetres (4.9 ft) high as a result of the ice floor lowering.[10]