joints

Compton Peak: superb columnar jointing

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After my talk Wednesday night to the Potomac Appalachian Trail Club, I got an email from PATC member Tom Johnson, with an extraordinary photo attached. It shows an exceptional outcrop of the Neoproterozoic Catoctin Formation, exposed atop Compton Peak in northern Shenandoah National Park, Virginia. The outcrop features enormous, well-preserved cooling columns from these ancient … Read more

Bookshelfed mullions from Norway

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Elizabeth Eide of the National Academies shared this image with me a year or two ago, when she gave a talk on Norwegian geology for the Geological Society of Washington. Those are some bookshelfed gneiss layers sandwiched between upper-left and lower-right carbonates. The “bookshelfing” refers to the numerous parallel brittle fractures along which the gneiss … Read more

Roadside wonders of Route 287

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Northern Colorado’s route 287 connects Fort Collins, Colorado with Laramie, Wyoming. Along its length, it displays roadcuts into Archean-aged basement complex. Two of these outcrops are featured in this post: one metamorphic (mostly), and a second igneous (mostly), with some intriguing polka-dotted plutons.

A Burgess plume

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At the Burgess Shale this summer, it wasn’t all fun and fossils. I also saw a lovely, distinctly feather-shaped plume: This is an example of plumose structure – the subtle branching micro-topography that forms on the surface of a joint as the fracture propagates out from its origin. The more obvious “rib” that is perpendicular … Read more

Graded bed from the Billy Goat Trail

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Spotted this one Monday on the newly-rerouted section of the Billy Goat Trail’s Loop A, in Chesapeake and Ohio Canal National Historical Park. This graded bed was deposited as a turbidite in the Iapetus Ocean, sometime in the time-frame of 700 to 460 million years ago. It was metamorphosed 460 Ma during the late Ordovician … Read more

Block Mountain basalt flow

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This year, for the first time ever, I took students to map at Block Mountain, a classic field camp mapping site near Dillon, Montana. Here’s a quick look (enlarge it by a gazillion-fold by clicking through) of some columnar jointing in the Eocene Block Mountain basalt flow, a paleo-drainage turned mountain through the miracle of … Read more

Friday fold: a granite dike

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This Friday, I give you a fold from the shores of the Rockfish River, south of Charlottesville, in Virginia’s Blue Ridge basement complex, and just down the road from the Lawhorne Mill High Strain Zone. The fold distorts (and improves) a felsic dike cutting the darker granite of the basement. You can make this (stitched … Read more

Some cool rocks from DGMR’s rock garden

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A brief tour of some cool rocks, shown in close-up, from the rock garden at the Commonwealth of Virginia’s Department of Geology and Mineral Resources in Charlottesville, Virginia, presented as a follow-on to the gigapan of the garden shown a month ago. The post features close-ups of plumose structure in slate, epidote slickensides, and graded bedding in ancient rhythmites.

Plumofare Gap

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I already mentioned the “paddle hackle” that I saw on the field trip I took to Thoroughfare Gap in February. Well, this week I went back out to Throughfare Gap twice, once with a student and once with my fianceé and a friend of ours. I saw cool new plumes both times, decorating joint surfaces … Read more

Two joints

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Stopped at Sideling Hill, Maryland, a few weeks back with my three Honors students, on our way to Pittsburgh for the northeast/north-central GSA section meeting. Robin took this photo of me with some sandstone beds that reveal two nice examples of joint anatomy, complementary in their structure: First focus in on the area right of … Read more

Plaster joint

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Here’s a joint extracted from gelatin during this year’s GMU structural geology “Make a Joint” exercise: A soda bottle full of congealed gelatin serves a “rock.” We then use construction clamps to impart a stress field to the gelatin bottle. Into it, we inject fluid plaster of Paris. The extra pore fluid pressure causes a … Read more