The Sharon Conglomerate is a Lower Pennsylvanian formation composed of sandstone and conglomerate which forms, according to Cushing et al., "disconnected patches or outliers that cap the highest hills... these outliers stand boldly above the surrounding country" due to its resistance to erosion. The Boston Ledges are the most noteworthy example. As the Mississippian shale underneath is washed away, huge blocks of the Sharon result from the settling. As Cushing et al. explain, "frost action aids in pushing these blocks apart, cracks are widened into caves, and a tangle of blocks results, separated by passages of uneven widths."