Field studies of air bubble and clathrate inclusions in the NGRIP ice core reveal two stages of clathrate formation. In the transition zone translucent primary clathrates with rough surfaces and irregular shapes, formed from individual bubbles, split up and form smaller specimens. Deeper down in the transition zone (~1100 m), clathrate metamorphosis sets in, giving rise to the formation of transparent secondary clathrates, with smooth and regular, often polyhedral or faceted shapes. Rod-like crystals of great length (~2 mm) often show signs of separation into smaller units that fragment into smaller specimens. These findings imply that the simplified picture of one bubble forming one clathrate has to be modified. ¿ 2001 American Geophysical Union