Last week was field trip week for me. I led trips to the Billy Goat Trail on Tuesday and Thursday, and to Washington, DC, on Saturday.

On the Physical Geology field trip to the Billy Goat Trail, we saw rocks like amphibolite, metagraywacke, and migmatite:







Hope and Ana checking out the migmatite:


The group poses with the migmatite, to show how close anatexis is to their hearts...


Jane examines lamprophyre in a weathered-out dike:


Noting the characteristics of metagraywacke:




Traversing 'Pothole Alley'... Joel looks chilly...


Our lunch spot... Alex pretends to dive into the Potomac River...


Traversing 'The Traverse':


On the Historical Geology field trip to DC on Saturday, we were amused to find a jack-o-lantern that had facial hair resembling mine...



But that's not all! We also saw some geology. While you can get a more complete picture at my "DC Rocks" webpage, I'll post a few new photos of new outcrops here...

Here's a nice slab of granite (very angular) set in metagraywacke matrix (metamorphosed accretionary wedge complex)...


Here's two members of the Georgetown Intrusive Suite, showing the (earlier) gabbro stoping xenoliths into the (later) granite:


I love field trips. I love seeing my students light up at being outside, at getting a handle on the stuff we talk about all semester in class. I think field trips are super duper important.

Labels: dc, field trips, granite, igneous, maryland, metamorphism, nova, piedmont, teaching, xenoliths

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

Today: I offer some photos I took in Boulder Canyon, Colorado, in June. These are all igneous rocks exposed in the Precambrian 'basement' rocks, brought to the surface by the Laramide Orogeny.

Directions: Drive to Boulder; go west up the main canyon into the Rocky Mountain Front Range.

Location map:


Granite pegmatite:


Contact! Granite pegmatite meets granodiorite:


Contact! Granite dike cutting across granodiorite (with one small mafic xenolith):


Contact! Mafic xenoliths afloat in granodiorite:


Put the previous two pictures together, and what do you get? My favorite outcrop of the whole excursion... Contact contact! A granite dike cutting across mafic-xenolith-bearing granodiorite. This would be a good practice photo for introductory level students to establish relative ages of the three different rocks shown:


Contact! More prosaic, but high-contrast... Granite meets basalt:


Epidote vein (Without any good reason, I love the color of epidote):


My Prius parked on the side of Boulder Canyon Drive:

Labels: basalt, colorado, granite, primary structures, prius, travel, xenoliths