A sequence of Lower Devonian rocks at Highland Mills, Orange County, includes (in ascending order) the Central Valley sandstone (new name), the Connelly conglomerate, the Highland Mills member (new name) of the Esopus formation, the middle member of the Esopus formation, the Woodbury Creek member (new name) of the Esopus formation, and the Kanouse sandstone. The Central Valley sandstone is of late Helderberg or Oriskany age, the Connelly conglomerate is of Oriskany age, the Highland Mills member is of post-Oriskany and pre-Woodbury Creek age, the middle member of the Esopus formation is unfossiliferous, the Woodbury Creek member is of post-Highland Mills and pre-Onondaga age, and the Kanouse sandstone is of Onondaga age. The articulate brachiopod fauna of 39 forms and 2 new genera, is described. The fauna of the Highland Mills member is correlated with that occurring in the lower part of the Gaspe sandstone. Emphasis is placed upon the recognition of stratigraphic breaks by study of the phylogeny of closely related genera present in the beds above and below a suspected break. The developmentally and stratigraphically intermediate faunas of the Highland Mills and Woodbury Creek members lead to the recognition of an Esopus stage in the Early Devonian of the Appalachians.