A couple of days ago, I asked for someone to tell me the geologic history of this boulder, in correct chronological order. To make it easier, I labeled the relevant rock units with letters. I promised that the first person to post the correct sequence of events in the comments would win a GEOLOGY ROCKS bumper sticker.



From first to last, the correct sequence of events is X, D, R, M, F.

Thomas Donlon got it right! Congratulations, Thomas -- I'll mail you a bumper sticker.

So let's delve into more detail: what actually happened with this rock?

First, a mafic source rock was weathered, generating chunks of rock "X." Then those clasts were mixed in with a bunch of sand and mud to generate the graywacke that makes up most of the boulder. This was later metamorphosed (not shown with a letter) to generate rock "D." Later, rock "D" with inclusions of "X" was split open, and granitic magma intruded into that fracture to make the dike labeled as "R." Later still, another cross-cutting event took place, cutting across everything that had come so far, to generate the vein of milky (hydrothermal) quartz labeled as "M." Finally, these rocks were uplifted and exposed, and various fractures, including "F," liberated this boulder from its source area. Now it is free, adrift on the Chain Bridge Flats, and posing for geologists. The final event was me discovering and gracing it with a quarter before snapping its portrait.

Labels: contest, dc, geologic time, piedmont