Mapping tips- differential weathering

When mapping a moderately weathered area, it is important to keep in mind that preservation-based selection effects may strongly bias the observable outcrop. Figure 1 shows some dolomite subcrop (with a silicified stromatolite) in red soil.

Figure 1. Dolomite subcrop with silicified stromatolite. Hammer for scale.

Since dolomite (plus or minus alteration) is the only rock that crops out in this area, it would be tempting to map the area as dolomite. However, as this historical digging shows (figure 2), the rock in the area actually consists of interlayered dolomite, shale, and dololutite. But the shale and dololutite are friable, so it is only the hard, massive dolomites (or silicified fossils) that are present in natural outcrop.

Figure 2. Historical diggings showing actual stratigraphy. Massive dolomite is confined to thin layers, and the dominant rock is shale and dololutite. Lizard for scale.