A 300 m succession of non-marine clastic rocks, herein termed the Idlorak Formation, lies in a graben on the Precambrian basement of Somerset Island, Arctic Canada. The indigenous microflora is probably late Cretaceous or Tertiary in age. Abraded fish remains, reaching 65 cm in length, include arthrodires, other placoderms, and crossopterygians, which were probably derived locally from former outcrops of Upper Devonian rocks.Eighty percent of the Idlorak Formation is well-sorted quartz sandstone; the remainder is shale, siltstone, and minor conglomerate. Abundant large-scale cross-stratification indicates fluvial transport from the north, and the presence of Lingula suggests a marginal coastal environment such as a delta. The largely arenaceous Griper Bay Formation and equivalent formations, now nearly 400 km to the north, may have formerly extended southward to provide the bulk of the source material.