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The Supai Formation of the Grand Canyon region is raised in rank to Supai Group and divided into four formations. Widespread surfaces of erosion and thin conglomerate beds separate these formations which range in age from Early Pennsylvanian to Early Permian. From oldest to youngest, they are named the Watahomigi, Manakacha, and Wescogame Formations and the Esplanade Sandstone. The type sections of these formations are in Havasu Canyon in the vicinity of Supai village. INTRODUCTION: FIELD TERMINOLOGY AND NEW NOMENCLATURE Probably the first geologist to carefully consider subdivision of the Supai Formation into logical, readily recognizable units was F. E. Matthes of the U.S. Geological Survey. In making a topographic map of the Bright Angel quadrangle in 1902-3, he noticed that the red beds of this formation, together with the overlying Hermit Shale, characteristically eroded into three prominent cliffs, interspersed among four welldeveloped slopes. The discrimination of these topographic units was called to my attention by Matthes in the 1930's in connection with my studies of the Supai Formation. A "field classification" that proved useful in recording data at that time and in all subsequent work resulted. Subdivision of the Supai on the basis of topographic expression, used during the present field investigation, recognized the following units: Upper cliff unit, locally includes a receding ledge unit (Esplanade cliff) Upper slope unit (divided by conglomerate into two parts) Middle cliff unit Middle slope unit Lower cliff unit Lower slope unit These units can be traced throughout the Grand Canyon, and they differ remarkably little from one end to the other, although, in general,

The Supai Group; subdivision and nomenclature