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.