Section across Keweenawan lavas at Duluth, Minnesota
INTRODUCTION STATEMENT OF UNDERTAKING The northwest coast of Lake Superior in Minnesota presents an almost unbroken line of rock exposure. It forms the southeast boundary of a great half-moon 1 of Keweenawan rocks whose convex margin points northwest. The elongated structural basin of Lake Superior is truncated by the shore line in such a way that the latter trends more or less parallel to the strike of the bedded rocks near the middle, but toward the ends it intersects the strike. Over that portion of the shore line covered by this study namely, from southwest of Duluth to Two Harbors (Fig. 1), a distance of about 40 miles, the shore intersects the strike of the east- to southeastward-tilted Keweenawan strata at angles of from 50 to 15 degrees, exposing the eroded edges of the beds along the lake front (Pl. 4, fig. 1). This makes it possible to run a linear . . .