This cover contains structure contours of the elevations of the unconformity at the Cambrian-Precambrian boundary. These data were contributed by Glenn Bear of Indiana University as part of the work on his doctoral thesis. The southern part of the map was produced by James Drahovzal of the Kentucky Geological Survey and merged with Bear's map of the rest of the map area and beyond. Use of public and proprietary seismic reflection and well data were used to define the structure. Units are in thousands of feet below sea level. A prominent east-west structure in the southern half of the map area is the western end of the Rough Creek Graben. At its deepest point the unconformity is more than 30,000 ft (9,100 m) below sea level. Faulted strata at this depth mean that the Rough Creek Fault System penetrates to hypocentral depths. At the western edge of the map area, the graben bends to the southwest into the Reelfoot Rift and becomes more shallow. North of the Rough Creek Graben, and extending northward along the Wabash River, is the Wabash Valley Fault System. This fault system cuts an elliptical low in the unconformity. Some of the structure contours show horizontal offsets at depths that are opposite to the offsets expected from the known normal slip on the faults. Some authors interpret this as evidence of strike slip motion on these faults. This cover contains all the contours including the extents outside the map area. "bcontours" is clipped to the map area.