Ice structure was determined from an analysis of photographs of vertical thin section photographs. As the plot below indicates, there was a preponderance of granular ice. Indeed 65% of the cores were more than half granular ice and approximately 10% of the cores were completely granular.

Summary of ice types determined from the cores taken for crystallographic analysis.

 
In this analysis granular ice is defined to include snow ice in addition to frazil, which comes in a number of textural variants. These include frazil of variable grain size, sometimes enclosing larger crystal plates, plus platelet ice and banded ice. A necessary condition of frazil is its nucleation and crystallization occurring freely in the water column. This mode of formation certainly explains the origin of the smaller grains or crystals of frazil as well as the large plate crystals. Banded frazil occurs as plates which, following nucleation, are advected upwards to the underside of an existing ice sheet. The plates generally orient themselves in a way that favors c-axis vertical orientation. The precise mechanism of nucleation of platelet ice is still being debated. One view based on observations in McMurdo Sound (Gow, personal communication) is that platelet ice originates by direct attachment of dendrite crystals to the underside of an ice sheet where subsequent freezing of the interstital water causes the whole mass of crystals to solidify. In a way platelet ice can be considered transitional to congelation ice.