4. The Geologic Setting

Ancient Volcanism . . .
If you've ever driven from Great Falls to Helena, you have driven through what's left of the ancient volcano. The part of the journey from Cascade to Craig goes right through the volcano which provided the magma for Crown Butte and other laccoliths in the neighborhood. The shaded area marks the extent of the ancient volcano, and the river shown on the map is the Missouri. Its headwaters are about 100 miles south of here. Crown Butte is not labeled, but it is the red spot about 12 km south-southwest of Simms. (km x .62 = miles)

The "Connection" . . .
The map shows the relationship between the dikes (blue lines), the laccoliths, and the volcanic center. The blue lines show where dikes are exposed as ridges at the surface. The pattern of lines on the map shows how the dikes connected the laccoliths (buttes) to the source of the magma (the ancient volcano). This "connection" is especially clear with Shaw Butte and Square Butte. Radiometric dating indicates that the volcano was active about 75 million years ago.

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5. The Plumbing of a Central Montana Laccolith