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If the attitude of a rock unit is known, it is usually possible to determine its approximate stratigraphic thickness from a geologic map. If a unit is steeply dipping, and if its upper and lower contacts are exposed on flat or nearly flat terrain, then the thickness is determined from the trigonometric relationship.

Figure 2.17 Neogene units in the Bree Creek area. Tg = Conglomerate; Thd = Sandstone; Tr = Tuff.

where t is stratigraphic thickness, h is horizontal thickness (map width), and δ is dip (Figure 2.18).

Figure 2.18 Trigonometric relationships used for determining stratigraphic thickness t in flat terrain from dip δ and map width h.