Origin of the Blue Rocks Block Field and Adjacent Deposits, Berks County, Pennsylvania

Abstract

Blue Rocks (40°36′ N., 75°55′ W.) is a half-mile-long block field on the south slope of Blue Mountain, 3 miles northeast of Hamburg, Pennsylvania. The block field consists of angular blocks of Tuscarora Quartzite 4 inches to 20 feet long. The blocks were derived from strongly jointed quartzite cliffs on Blue Mountain and have moved downslope over the Martinsburg Shale. On two sides of the field of open blocks are forested rubble deposits that contain boulders equivalent in size to those in the block field, but with fine material in the interstices. The rubble adjacent to the block field occurs as gently sloping, step-like terraces with fronts 10 to 30 feet high and slopes of 18° to 22°. The block field is separated from the source cliffs by about a half mile of rubble covered by mature forest. Tabular blocks in the block field have an imbricate structure and describe a series of lobes in which the blocks dip steeply upslope. The blocks are crudely sorted vertically, with coarser material at the surface. Locally, horizontal sorting on a small scale is also apparent. Blue Rocks and the adjacent rubble deposits closely resemble solifluction sheets or terraces on the Seward Peninsula, Alaska. The lack of fine material in the block field probably resulted in part from the flushing of fines after downslope movement ceased. The block field is older than the mature forest on the rubble between the block field and the source cliffs. It was probably formed by solifluction or creep in the periglacial climate of the Wisconsin glaciation, after which removal of fines occurred.