When the Canadian Government decided to relocate the town of Aklavik in the delta of the Mackenzie River, Northwest Territories, the search for a new site involved detailed site investigations of four prospective locations. One was on the gently sloping surface of an alluvial fan between the Richardson Mountains and the western rim of the Mackenzie Delta. Predictions from surficial evidence and from a detailed study of aerial photographs were invalidated when drilling in the perennially frozen ground disclosed at least 40 feet of organic silt and no sand or gravel as had been expected. The adjacent mountains of Cretaceous age supply materials eroded from sandstone and shales. The action of frost probably induces mechanical disintegration. Turbulent stream flow continues the erosion process, which appears to be still further aided by the annual growth of fast-growing sedges and grasses on the surface of the stream meanders. The short but warm growing season leads to rapid decay of this grass cover; this combined with the annual layers of stream-bed material results in the fans being composed predominantly of organic silt with only minor quantities of coarse-grained material.