A better knowledge of boundary conditions near a grounding line is critical for understanding the behavior of ice shelves and floating glaciers. We show here thatsignificant information can be gained from a high-resolution isotopic and textural studyof marine ice accreted at the bottom of a floating glacier near its grounding line. Two different types of marine ice have been found. Type 1 is bubble-and debris-free icewith properties which, we believe, can be explained by intrusion of brackish water inopen basal fissures. Closing of the fissures by progression of a freezing front from thesides is precluded, and filling by frazil ice is favored. Type 2 is made of thin, clear iceand debris layers which are thought to have formed when a subglacial water-filledsediment enters into contact with seawater and is subjected to freezing under a double-diffusion process. The paper also stresses that in a δD-δ18O diagram the alignment of marine ice samples on a mixing line does not necessarily imply a mixture of continental water and seawater in varying proportions.