There are three types of subglacial channels, depending on such factors like glacier movement, bedrock topography and lithology:
N-channel or Nye-channel: incised in the underlying bedrock.
R-channel or Röthlisberger-channel: incised in the ice (and so not found in "fossil" form).
C-channel or Clarke-channel: partly incised in the bedrock and ice, a combination of the formerly mentioned types.
http://rockglacier.blogspot.com/2009/09/geologists-who-say-nye.html