Continental-type glaciers, which covered the Montréal area, are known to have been responsible for bedrock deformation related to compressive forces. On the other hand, here we report cave systems hundreds of metres long that were caused by tension and shearing of the same bedrock. These voids were created in a low-relief area and in thin, interbedded limestone and shale. Their geometrical characteristics suggest that at least the uppermost 10 m of this limestone was broken up during middle Wisconsinan time and that this intersliding of stratified rock masses continued until the late Wisconsinan. Except for rare collapses, these voids, of unknown sizes until now, show almost no connection with the superficial geomorphology.