The Ordovician Chazyan series succeeds the Canadian series and underlies the Blackriveran series. In the type region of the Champlain Valley the series is separable into the Dayan, Crownian, and Valcourian stages, distinguished by faunal criteria. Thickness approaches 1,000 ft. as a maximum, diminishing by overlap, offlap, and convergence. Local variations are so great and outcrops so distributed that regional isopach maps are only suggestive. Each stage has several lithologic facies. Reefs are prevalent in sections having minimum thickness, contrasting with argillaceous calcisiltstones of the thicker sections. The reef-building organisms include some of the earliest corals and bryozoans, as well as sponges and stromatolites. The reef belt lay between lowlands of the continental interior and deeper water of western New England.