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OUTSTANDING GEOLOGIC FEATURES OF PENNSYLVANIA MONROE BORDER FAULT BUCKS COUNTY Location Route 611 and Rehnenberg Rd., Bucks Co., Durham Twp., lat: 40.5723, lon: -75.1925, Riegelsville 7.5-minute quadrangle The site has been designated a National Natural Landmark. Above, view of Triassic rocks dipping to the northwest and plaque, which reads “This site possesses exceptional value as an illustration of the nation’s natural heritage and contributes to a better understanding of man’s environment.” Right, a geologic cross section looking east. Geology The Monroe Border Fault (Triassic Border Fault), and the adjacent rocks, together provide a textbook example of Pennsylvania’s long geologic history. The area has undergone multiple episodes of deposition, mountain building, and erosion. Immediately north of the border fault are tightly folded and faulted rocks, which are from a series of complexly-folded thrust sheets pushed sideways about 440 to 500 million years ago. This is the Reading Prong section of the New England physiographic province. It is composed of metamorphosed sedimentary rocks of dolomite and quartzite that are over 500 million years old, and folded-in Precambrian gneisses (or granitic rocks) that are some of the oldest rocks in the state at over a billion years old. Each unit had its own geologic history before being pushed over Cambrian-age limestone. After the Alleghanian mountainbuilding event (about 290 million years ago), a down-warped basin formed in fairly high terrain as Africa began its separation from North America about 250 million years ago. Sediment poured in during the Triassic Period as the basin deepened into what is now the Gettysburg-Newark Lowland physiographic section. Coarse-grained sediments accumulated along the basin edges. Rivers laden with sediment built alluvial fans. The climate was generally warm but variable; dinosaurs passed through the region while lakes came and went. About 200 million years ago, diabase sills and dikes intruded the basin. Normal faulting and some folding then developed as the basin tilted to the northwest. The border fault marks the boundary between these geologic remnants. Millions of years of erosion have unearthed evidence of its deep geologic history. Recommended Reading Inners, J. D., 1980, Nockamixon State Park, Bucks County—Rocks and joints: Pennsylvania Geological Survey, 4th ser., Park Guide 14. For information regarding outstanding geologic features, contact the Pennsylvania Geological Survey: Bureau of Topographic and Geologic Survey, 3240 Schoolhouse Rd., Middletown, PA 17057 717-702-2017 www.dcnr.state.pa.us/topogeo Nockamixon State Park is located just five miles south. S. Reese, 2013