With structural and magmatic trends paralleling the length of the Transantarctic Mountains (TAM), the Ross orogen formed along the paleo-Pacific margin of Gondwanaland during the Neoproterozoic-early Paleozoic. Transecting the TAM, Byrd Glacier marks a major discontinuity in the geology of the Ross orogen completely across the 100 km of its exposure. To the north of Byrd Glacier the lithology is entirely crystalline, including plutonic Granite Harbour Intrusives, and Horney Formation, upper amphibolite-grade gneisses (including migmatitic and garnet-bearing fine grained varieties), quartzitic paragneisses, and Ca-silicate rocks, lacking marbles. To the south of Byrd Glacier, most rocks belong to the deformed but unmetamorphosed Byrd Group, including Early Cambrian Shackleton Limestone, overlain by the clastic Dick Formation and Douglas Conglomerate. Immediately south of Byrd Glacier in a 6 x 15 km area around Mt. Madison, is a unique locality of amphibolite-grade metamorphic rocks, the Selborne Marble of Skinner (1965). A goal of our recent study south of Byrd Glacier was to document the Selborne Marble and to determine its correlation with other units in the region. We found that Selborne Marble is in fact composed of approximately equal portions of marble and of biotite schist, which occur in areas primarily exclusive of the other, marble to the east, schist to the west. The units interfinger at an apparently conformable contact. Within the schist close to the contact is a thick unit of greenstone (actinolitic hornblende-plagioclase metabasite). Metaconglomerate also was found within the schist at the western end of its exposure. Based on a one to one match of the lithologies and stratigraphy of Selborne Marble with Byrd Group (including pillow basalts in the lower portion of Dick Formation) we correlate the two sequences. Eliminating the possibility that the Selborne Marble has stratigraphic equivalents to the north of Byrd Glacier reinforces the notion that Byrd Glacier marks a fundamental discontinuity orthogonal to the main tectonic fabric of the Ross orogen. So what to make of the Byrd Glacier discontinuity? Inherited scar from the breakout of Laurentia? Terrane suture or paleosubduction zone? Post Ross strike-slip offset? Or simply markedly different degrees of exumation following the Ross orogeny?