joints

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