At the edge of the intrusion

Mountain Beltway reader Greg Willis attended my colleague Ken Rasmussen’s Triassic Rift Valley field course last weekend, and sent me this photo of the view inside the Luck Stone diabase quarry in Centreville, Virginia:

Here’s an annotated version:

Both photos are enlargeable by clicking on them (twice).

This quarry chews into rock right along the contact between a mafic igneous intrusion and lake sediments that formed when water pooled in a low-lying continental basin that formed during the breakup of Pangea. This rift valley, the Culpeper Basin, is just one prominent basin in a whole series of Triassic grabens and half-grabens that run through the Piedmont north and south of here, including all the way to the Bay of Fundy.

A similar environment can be seen today in east Africa, where a modern rift valley hosts similar lake deposits and mafic igneous rocks:

If you were to drop maybe half a kilometer below the surface of the Afar region, you’d see a similar situation to the one that produced Greg’s quarry photo ~200 million years ago.

Visiting the Centreville quarry is by permission of the Luck Stone corporation only; the best way to see it is by signing up for Ken’s course the next time it rolls around!

Triassic rifting in the Capitol

My girlfriend’s mom was in town in January, and we took her down to visit the Capitol Building. The tour had a good bit of history, but definitely missed the opportunity to talk geology. I was particularly struck by the columns in the Hall of Statuary:

Close up of one column, with my hand for scale:

That’s the Leesburg Conglomerate, a Triassic-aged deposit found in the western part of the Culpeper Basin of Virginia. (Technically, it’s “the Leesburg Member of the Balls Bluff Siltstone.”) The photos I showed readers in May 2008 were from the east side of Route 15, just north of Leesburg itself. The Culpeper Basin is a failed rift valley from the time of Pangea’s breakup. I say “failed” in the sense that it failed to become an ocean basin like the Red Sea or the Labrador Sea. While it may have failed to rend the metamorphic rocks underlying Reston, Annandale, and D.C. from the North American continent, it succeeded in accumulating continental sediments for two periods of geologic time, preserving a detailed record written in siltstones, conglomerates, basalt flows, diabase intrusions, dinosaur footprints and fish fossils.

Among the strata that the basin accumulated, the Leesburg Conglomerate stands out as the real rock star. It’s a gorgeous looking rock, a poorly-sorted and well-oxidized mishmash of (mainly) limestone chunks derived from the weathering of the young Appalachian Mountains. Visually striking as it is, it’s not surprising that someone tried to use it as a building stone. However, it’s not well-suited to being sculpted. Rumor has it that after countless episodes of pebbles popping out of otherwise pristine, finished columns, the column-carver swore he would never touch this particular stone again. To my knowledge, the Capitol’s Hall of Statuary is the only place in the world where the Leesburg Conglomerate has been used as a building stone.