Facies maps based on numerical stratigraphic data (percentages, relative thicknesses, and ratios of lithologic components) are relatively new in the literature. Many such maps represent experiments in methodology, and the wide variety of patterns displayed on the maps presents many problems of interpretation. In part, interpretation of facies maps depends upon the scale of the map, the magnitude of the stratigraphic interval, the geographic area involved, and the relative rate of change of the facies in the map area. In general, two types of maps are recognized: regional maps , which show broad trends over large areas. and local maps , which show greater detail in smaller areas. The distinction is partly on the basis of map scale, magnitude of stratigraphic unit, and density of control. The isopach map of the stratigraphic unit is used as a base for the facies map, and interpretation of the facies patterns is closely related to interpretation of the isopach lines. The concept of "facies strike" and "isopach strike" are useful in map interpretation, inasmuch as they provide a basis for testing similarities or divergences between these two kinds of strike and the more familiar structural strike of the rocks. The present paper reviews some of the principles of facies map construction and attempts to develop principles of map interpretation. These principles are based mainly on relations between the kinds of strike, on rates of change, and on an evaluation of the erosional modification of the original patterns.

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