ABSTRACT This field guide presents a one-day trip across the northern Virginia Blue Ridge geologic province and highlights published geologic mapping of Mesoproterozoic rocks that constitute the core of the Blue Ridge anticlinorium and Neoproterozoic cover-sequence rocks on the fold limbs. The guide presents zircon SHRIMP (sensitive high-resolution ion microprobe) U-Pb crystallization ages of granitoid rocks and discusses the tectonic and petrologic evolution of basement rocks during the Mesoproterozoic. U-Pb data show more of a continuum for Blue Ridge Mesoproterozoic magmatic events, from ca. 1.18–1.05 Ga, than previous U-Pb TIMS (thermal ionization mass spectrometry)-based models that had three distinct episodes of plutonic intrusion. All of the younger dated rocks are found west of the N-S–elongate batholith of the Neoproterozoic Robertson River Igneous Suite, suggesting that the batholith occupies a fundamental Mesoproterozoic crustal boundary that was likely a fault. Narrow belts of paragneiss may represent remnants of pre-intrusive country rock, but some were deposited close to 1 Ga according to detrital zircon U-Pb ages. For late Neoproterozoic geology, the guide focuses on lithologies and structures associated with early rifting of the Rodinia supercontinent, including small rift basins preserved on the eastern limb of the anticlinorium. These basins have locally thickened packages of clastic metasedimentary rocks that strike into and truncate abruptly against Mesoproterozoic basement along apparent steep normal faults. Both basement and cover were intruded by NE-SE–striking and steeply dipping, few-m-wide diabase dikes that were feeders to late Neoproterozoic Catoctin Formation metabasalt that overlies the rift sediments. The relatively weak dikes facilitated the deformation that led to the formation of the Blue Ridge anticlinorium during the middle to late Paleozoic as the vertical dikes were transposed and rotated during formation of the penetrative cleavage. This field trip generally follows the GSA guide published in GSA Field Guide 57 (available at https://pubs.geoscienceworld.org/gsa ): Burton, W.C., and Schindler, J.S., 2020, Proterozoic and Paleozoic evolution of the Blue Ridge geologic province in northern Virginia, USA, in Swezey, C.S., and Carter, M.W., eds., Geology Field Trips in and around the U.S. Capital: Geological Society of America Field Guide 57, p. 1–19, https://doi.org/10.1130/2020.0057(01) .

Abstract Most stories about the early days of petroleum geophysics start with the formation of the Geological Engineering Company. 2 We mentioned in the prior chapter that four talented men—W. P. Haseman, J. C. Karcher, E.A. Eckhardt, and Burton McCollum—worked together at the U.S. Bureau of Standards in 1917 to develop improved detectors of blast waves generated by enemy artillery. While working there, Haseman discussed with Karcher his belief that reflected sound waves could detect potential petroleum-bearing structures. After resuming teaching at the University of Oklahoma, Haseman wrote Karcher, then finishing his Ph.D. degree in physics at the University of Pennsylvania, to ask whether he would like to form a seismic exploration firm upon graduation. Karcher was enthusiastic and immediately began working on a patent application for the concept. They also invited Eckhardt and McCollum to join them. Because McCollum already had eleven patents, in January 1919 he and Karcher jointly filed a patent application for “determining the contour of subsurface strata….” Actually, they prepared four separate applications, two on reflection methods and two on refraction methods. To stay solvent, Dr. Eckhardt and the new Dr. Karcher continued working at the Bureau of Standards and, during off-hours, built a three-trace recorder from an oscillograph and converted some radiotelephone receivers into electrodynamic geophones. With this crude gear, the world’s first seismic reflections were intentionally obtained on 12 April 1919 within a Maryland rock quarry. 3 The Geological Engineering Company, the first seismic contracting organization was then incorporated in Oklahoma in April 1920. By early