I led my "History Before History: The Geologic Saga of Washington, DC" tour twice this weekend as part of the twice-annual "Walkingtown, DC" weekend. The folks attending the tour both days were really cool, and were full of good questions. We covered the sedimentary origins of DC's rocks at the bottom of the Iapetus Ocean, their metamorphosis during Taconian mountain building, the intrusion of plutonic rocks, the erosion of those ancient mountains, the deposition of river gravels during the Cretaceous (together producing an unconformity), and the faulting of that unconformity sometime post-Cretaceous (probably Miocene). I'm kind of tired after all that geologic history, especially repeated twice in two days!



The photo above is of a boulder in Rock Creek Park showing all three members of the Georgetown Intrusive Suite, a series of igneous plutons that were intruded into the crust during late-Ordovician mountain-building. I like this boulder because it illustrates well two of the principles of relative dating: the gabbro must be older than the diorite, because there are xenoliths of the gabbro in the diorite (inclusions). You can't break off a piece of gabbro unless it already exists. The granite dike must be younger than the diorite, because it cuts across the diorite (cross-cutting relationships). You can't crack open diorite unless it already exists.

Just thought I'd share an informative little outcrop like this. Please ignore the white graffiti that mars the central part of the exposure. A pen at the top is circled to give a sense of scale.

I hope everyone had a relaxing weekend!

Labels: dc, granite, igneous, plutons, tourism, xenoliths