Abstract The Jefferson River Canyon area is on an important structural boundary between the Cordilleran thrust belt and the Rocky Mountain foreland (Fig. 1). The features described here reflect the tectonic history of that boundary from late Precambrian through late Cenozoic time. The area is in Jefferson and Madison counties. Most of the features described may be observed along U.S. 10 through the canyon and along the access road to the Lewis and Clark Caverns State Park (Fig. 2). Stops of particular interest discussed in the text are numbered and indexed in Figure 2 with mileages indicated from LaHood Park (bar, restaurant, and sometime gas station and motel) and/or from the intersection of U.S. 10 and the state park access road. The area is located in the center of the Jefferson Island 15-minute Quadrangle. Many of the features that can be seen across the Jefferson River from U.S. 10 (south side of river) are currently accessible via the abandoned track bed of the Chicago, Milwaukee, St. Paul, and Pacific Railroad. The track bed is easily traversed by most vehicles and may be accessed on the west from County Road 359 at Jefferson Island and on the east from Montana 287 at Sappington. The access route at Jefferson Island is privately owned and the owner (Vernon Shaw of Cardwell) should be contacted for permission.