Viking moderate- and high-resolution images along the northern highland margin were studied monoscopically and stereoscopically to contribute to an understanding of the development of fretted terrain. Results support the hypothesis that the fretting process involved flow facilitated by interstitial ice. The process apparently continued for a long period of time, and debris-apron formation shaped the fretted terrain in the past as well as the present. Interstitial ice in debris aprons is most likely derived from ground ice obtained by sapping or scarp collapse. Debris aprons could have been removed by sublimation if they consisted mostly of ice, or by deflation if they consisted mostly of debris. To remove the debris, wind erosion was either very intense early in martian history, or was intermittent, perhaps owing to climatic cycles. The location of debris aprons in two midlatitudinal belts can be attributed to atmospheric conditions that permit temperatures to be warm enough for ice to flow. Channels have minimum gradients of 5--8 m/km, which permits flowing ice to deform by dislocation creep at exceedingly low strain rates; alternately, ice-charged debris may have been buoyed by brines. By analogy with the fretted channels, ice could have flowed through the outflow channels if the atmospheric pressure was somewhat higher in the past and permitted ice to exist in the equatorial area.