Yerkes is a lunar crater near the western edge of Mare Crisium. In the past the interior of this crater has been almost completely inundated by lava, leaving only a shallow remnant of a rim above the mare. The rim is widest on the western and southern portions, and barely existent to the east, forming a thin curve in the surface. A low ridge runs from the north rim to Yerkes E in the north-northwest. (wikipedia entry)