Profiles of the underside of sea ice are rough and disorderly. No natural definition of the roughness elements is found; they have all sizes and shapes. This raises questions concerning the practice of identifying and counting pressure ridge keels. The profiles can be characterized by their power spectral density functions. It is found that the spectra vary approximately as (wave number)-3 at high wave numbers. This observation is related to other measures of roughness, including continuity, differentiability, and the fractal or Hausdorff dimension, and to the distribution of the drafts of local maxima. The latter is seen to correspond closely with the ice thickness distribution.