FIGURE 10.--Circular faults shown on the Geologic Map of the United States. A and B are associated with calderas, C and D with astroblemes. A, Yellowstone and Island Park calderas northwestern Wyoming and adjacent Montana. B, Calderas in San Juan Mountains, Colorado. C, Monson structure, central Iowa. D, Wells Creek Basin, west Tennessee. Contacts are the same as on the Geologic Map, but units are grouped in the legend.