As we have seen in "Introduction 1", diagram A (above) is a simple interpretation of the outcrops shown on the map to the left. A subsurface arrangement of formations that would produce the pattern shown in A is indicated in diagram B.

But many different patterns of inferred contacts may be drawn. Here is one example. Note how the inferred contacts (shown in yellow) together with the observed contacts (shown in black) separate the red and green formations observed in the outcrops. That is, the red and green areas are on opposite sides of the lines.

Now, areas underlain by the red and green formations are inferred and colored appropriately.

When the geologist is satisfied that the pattern of formations and contacts accurately reflects the actual geology, the distinction between observed and inferred contacts and formations is eliminated. The construction of the geologic map has been completed.

A subsurface arrangement of formations that would produce the pattern shown on the geologic map is indicated above.

Here is yet another pattern of observed and inferred contacts. Once again, the contacts separate the green and red formation, so that the green and red formations lie on opposite sides of the contacts.

Now, areas underlain by the red and green formations are inferred and colored appropriately.

Again, when the geologist is satisfied that the pattern of formations and contacts accurately reflects the actual geology, the distinction between observed and inferred contacts and formations is eliminated. The geologic map has been completed.

A subsurface arrangement of formations that would produce the pattern shown on the geologic map is indicated above. Here, the land surface is not flat. Wherever the land is low, the red formation is directly beneath the surface. Where it is high, the green is directly underneath.

To decide which map is most likely to be correct, more information is needed. Discovery of a new outcrop in a critical location; drilling a hole to identify subsurface materials; knowledge as to whether the land is flat or irregular; being able to see whether the formations are horizontal, vertical, or tilted at an angle - would help answer the question.

For example, if the rock underneath 'X' is partly or totally "green", then Map A cannot be correct. If the rock under 'Y' is "red", then neither Map A nor Map B can be correct. If the land is flat, then Map C cannot be correct. As more information is gained, more ideas can be shown to be wrong. They then can be modified or rejected.

If it turns out that the rock under 'X' is part "green", the rock under 'Y' is part "red", and that the land is flat, then neither Map A, B, nor C can be correct. So map D is proposed. The more 'testing' of this sort a map survives, the more likely it is to be correct. A map can be shown to be wrong or to be probable; it cannot be shown to be right. A map that survives testing over the years is a work of art.