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 that signifciant information can be gained from a high-resolution isotopic and textural study of 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 ice with properties which, we believe, can be explained by intrusion of brackish water in open basal fissures. Closing of the fissures by progression of a freezing front from the sides is precluded, and filling by frazil ice is favored. Type 2 is made of thin, clear ice and debris layer which are thought to have formed when a subglacial water-filled sediment 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. ¿American Geophysical Union 1995