Conglomerate Outcrop in Stream
August 31, 2004
Conglomerates are unique sedimentary rocks that form when rocks and pebbles become cemented together within in a mixture of finer sand or silt. Quartz pebbles are frequently found in conglomerates because of their hardness. The formation of conglomerates requires that sand, mud, clay, or silt be homogenized with larger rocks and pebbles. This most commonly happens in fast moving streams, ocean waves, landslides, or glaciers. Over millions of years, this mixture is compressed and hardens into solid rock. Conglomerates can contain virtually any size or type of rock or mineral mixed in virtually any clay, mud, sand or silt base. The hardness, color, and appearance of conglomerates varies accordingly. For reasons that are not entirely clear, conglomerates are most commonly found in the U.K., the eastern U.S. and in South Africa.
The conglomerate above was found in the Appalachian foothills of north-central Pennsylvania, U.S. A unique aspect of this photograph is that it depicts a fast moving stream slowly eroding a conglomerate outcrop, gradually reducing it back to the sand, silt, and quartz pebbles it initially formed from. The speed of the rushing water has separated the components of the conglomerate outcrop by particle size, forming a pebble streambed adjacent to the outcrop, with sand, and then silt, further downstream. As may be discerned on the photo, this outcrop seems to consist of very smooth quartz pebbles held in a sandstone. Wherever it formed must have been an area where the action of water would both wear the quartz smooth and homogenize the resulting pebbles and sand. Perhaps it was formed by the edge of an ancient sea or within a fast moving river -- the buried sedimentary remnants of which were later uplifted and subsequently exposed by the erosive action of this stream.