VIRGINIA DIVISION OF GEOLOGY AND MINERAL RESOURCES PUBLICATION 178 GEOLOGY OF THE MONTEREY QUADRANGLE, VIRGINIA Gerald P. Wilkes COMMONWEALTH OF VIRGINIA DEPARTMENT OF MINES, MINERAL AND ENERGY DIVISION OF GEOLOGY AND MINERAL RESOURCES David B. Spears, State Geologist CHARLOTTESVILLE, VIRGINIA 2011 (Revised April 2013) FRONT COVER: Trimble Knob in winter with Monterey Mountain in background. This landform, south of Monterey, is the remnant of an ancient volcano. CONTENTS Introduction......................................................................................................................................1 Stratigraphy…..…………………………………………………………………………................2 Paleozoic………………………………….......................……………………...........……2 Ordovician............…………….....……………………………………………………2 Beekmantown Formation……………………………………….....………2 Middle Ordovician limestone........................................................................3 Lower member…………...…………………………………..……3 Middle member……...................................………………….……3 Upper member….…................................…………………….……3 Dolly Ridge Formation…………………………….........................………4 Reedsville Shale and Oswego Sandstone..……………………...........……4 Juniata Formation ………………………………………………..…..……4 Silurian……………………………………………..……........................................5 Tuscarora Sandstone …………………………………………………....…5 Rose Hill Formation and Keefer Sandstone.………………....………..……5 McKenzie Formation…………………………………………...…….……5 Williamsport Sandstone…...........…...............................……….…….……6 Wills Creek Formation and Tonoloway Limestone…………….…….……6 Devonian………………………………………………………...........................…7 Helderberg Group …………………………………………………....……7 Ridgeley Sandstone.............. ........................………………………....……8 Needmore Formation and Millboro Shale…………………………....……8 Brallier Formation.........................................................................................9 Mesozoic and Cenozoic…………..................…………………………..............…9 Igneous rocks………………………………………………….…..........…9 Surficial deposits………………………………………………...........…..10 Structural Geology.........................................................................................................................11 Regional Setting..................................................................................................................11 Allgehany Plateaus province...............................................................................................11 Alleghany structural front.......................................................................................11 Valley and Ridge province................................................................................................11 Hightown anticline.................................................................................................11 Monterey syncline..................................................................................................12 Bolar anticline........................................................................................................12 Jack Mountain anticline.........................................................................................13 Bear Mountain fault................................................................................................13 Shenandoah Igneous Province...........................................................................................14 Economic Geology.........................................................................................................................14 Limestone and dolostone...................................................................................................14 Sandstone and quartzite.....................................................................................................15 Shale...................................................................................................................................16 Oil and Gas.........................................................................................................................16 Brucite................................................................................................................................16 Non-polishing aggregate....................................................................................................16 Soda feldspar.......................................................................................................................16 Precious minerals...............................................................................................................16 References cited..............................................................................................................................17 ILLUSTRATIONS Plate 1. Geologic Map of the Monterey quadrangle, Virginia Figures 1. Location of the Monterey quadrangle.........................................................................................1 2. Barren Rock on northern Jack Mountain......................................................................................2 3. Hightown Valley looking west from Monterey Mountain.............................................................3 4. Sinkholes in the middle Ordovician rock unit...............................................................................4 5. Devils Backbone seen from Bluegrass.......................................................................................5 6. Ripple marks in the Rose Hill.......................................................................................................5 7. A minor fault in the Williamsport Sandstone along State Route 642..............................................6 8. An unnamed sandstone bed in the Wills Creek Formation.............................................................7 9. Corriganville limestone outcrop along Strait Creek....................................................................8 10. The lowest part of the Ridgely Sandstone...................................................................................8 11. Millboro Shale containing concretions......................................................................................9 12. Basalt intrusive rock.................................................................................................................10 13. Landforms in the Hightown valley made from both alkalic and mafic rock..............................10 14. Volcanic Breccia outcrop.........................................................................................................10 15. Talus below Barren Rock on northern Jack Mountain..............................................................11 16. Alleghany Structural Front in Pendleton County, West Virginia..............................................12 17. View of Hightown Valley.........................................................................................................12 18. View of Bolar Valley.................................................................................................................13 19. Barren Rock on northern Jack Mountain...................................................................................13 20. Rockbridge Stone Products #1 quarry.......................................................................................14 21. Abandoned limestone quarry east of Monterey........................................................................15 22. Residual chert borrow pit.........................................................................................................15 23. Tuscarora talus at base of Devils Backbone................................................................................16 PUBLICATION 178 1 GEOLOGY OF THE MONTEREY QUADRANGLE, VIRGINIA Gerald P. Wilkes INTRODUCTION The Monterey Quadrangle is located in Highland County, Virginia and Pendleton County, West Virginia (Figure 1). Bedrock in the quadrangle includes Ordovician, Silurian, and Devonian carbonate and clastic rocks. These rocks were folded and faulted during the Alleghanian Orogeny in the late Paleozoic. Multiple igneous events during the Mesozoic and Cenozoic intruded the Paleozoic rocks with dikes, sills and plugs. Masking most of the bedrock is a mantle of soils of Pleistocene and Holocene age. A variety of economic rock deposits can be found in the quadrangle and others possibly exist. Charles Butts (1933, 1940) offered the first comprehensive geologic map and stratigraphic synthesis of the Monterey area at a scale of 1:250,000. Rader and Wilkes (2001) produced a 1:100,000-scale geologic map, which refined Figure 1. Location of the Monterey quadrangle. Butts’ mapping. Additional geologic mapping was conducted in the Bluegrass Valley portion of A geographic oddity within the quadrangle the Monterey 7.5’ quadrangle by Parrott (1948) can lead to confusion when describing the geology. and Tarleton (1948). Kettren (1970) mapped in There are two Jack Mountains in the southeastern the upper reaches of the Strait Creek area, paying portion of the quadrangle, and although they are special attention to the many igneous bodies in a single physiographic mountain range, they are that vicinity. Tso and Surber (2003) mapped formed by two different geological structures: a portion of the Monterey quadrangle, also in the Bolar anticline and Jack Mountain anticline. the Strait Creek section of the quadrangle. A To differentiate the two mountains throughout Virginia