 The McElmo Phase was a period in the late 11th and early 12th centuries, when major changes in ceramics and masonry techniques appeared in Chaco Canyon.Chacoans started using painted black-on-white pottery, and the masonry and layout of great houses built during the period, which was the last major construction era in the canyon, differs significantly from those built during the Bonito Phase (850 to 1140).Archeologists initially believed that the McElmo style was brought to Chaco Canyon by immigrants from Mesa Verde, but subsequent research suggests the developments were of local origin.