Figure 2.01. Contact Metamorphism by the Purcell Sill. White marble bands border an olivine diabase sill in the Helena Formation (Siyeh Limestone) in Glacier National Park, Montana, USA. Intrusion of the approximately 20m thick, diabase sill heated the host limestone producing a bleached zone (marble) by contact metamorphism. Click on the "Show Hres" button to see a closeup view. You can read more about the Purcell Sill on the National Park Service website and in this M.S. thesis by P.F. Mejstrick. The photographs of the Purcell sill were taken by Marli Miller and used with permission.