The time interval represented by the sub-Devonian unconformity in the eastern Rocky Mountains varies considerably. The Devonian rests on Cambrian rocks in the south, but farther north, a thin sequence of unfossiliferous dolomites of uncertain age intervenes between the Cambrian and Devonian strata in the front ranges along and north of Bow River. Farther to the west, the Devonian overlies a sequence of Ordovician dolomites and quartzites of varying thickness and age. An early Ordovician (Canadian) fauna, characterized by Bellefontia and other trilobite genera, occurs widely, affording a useful horizon marker. A late Ordovician (Richmond) fauna occurs in the so-called Halysites beds and other Ordovician horizons may be represented. Little or no angular di cordance is visible at the sub-Devonian contact and the magnitude of the break is evaluated from faunal and stratigraphic evidence. Study of the sub-Devonian unconformity may provide a clue to the loci of Devonian reef complexes.