In eastern Minnesota south of lake Superior the area known to be underlaid by the Keweenawan series comprises about 900 square miles. It borders the state of Wisconsin for at least 100 miles, and lies almost wholly within the valley of the Saint Croix river.
The northern limit of the district is near latitude 46° 25′, at the boundary line between Minnesota and Wisconsin; the southernmost point exposed is in the lower Dalles of the Saint Croix near Franconia (latitude 45° 15′), although the lava flows of the southern exposures stretch beneath the Cambrian formations southward beyond Stillwater. Westward the Keweenawan stops abruptly along a fault line which nowhere passes beyond longitude 16 degrees west of Washington. The total length of the area is nearly 70 miles, and greatest width not more than 30, with an average of 12 to 14 miles.
SURFACE FEATURES . . .