It’s Gneiss to be Old

William & Mary is celebrating its 319^th birthday this weekend.  For an institution of higher learning in the western hemisphere, 319 years certainly qualifies as venerable.  Although what qualifies as old depends on your perspective; geologists typically take a long view on time.  It’s easy to do when you consider that the Earth’s history stretches out to well over four billion years.

Consider the photograph below, a little snippet of bedrock cropping out high above Harris Cove in the Blue Ridge Mountains of western Virginia.  The rock is a gneiss (pronounced- nice!), a curiously colorful granitic gneiss.  This is an ancient rock, it crystallized and cooled into a granite, far below the Earth’s surface, some 1.15 billion years ago.  It was later metamorphosed and transformed to a coarsely-foliated gneiss.  By the late Neoproterozoic (~550 million years ago) the rock was exposed at the surface and then buried beneath a sequence of lava flows and sedimentary deposits.  At some point the rock underwent another transformation and the iron-bearing minerals were altered into a new mineral, epidote—a distinctive greenish silicate, while the feldspar turned pink.  Indeed, it’s an old rock with quite a history.

Outcrop photo of granitic gneiss in the Virginia Blue Ridge. K-feldspar is pink, quartz is blue-gray, and epidote is green. Note foliation from upper left to lower right and distinctive coin for scale.

Field geologists take oodles of rock pictures and invariably we include some marker to provide ‘scale’ for the picture. I’ve used rock hammers, lens caps, pocketknives, and even doughnuts to provide scale. Coins are a time-honored standard, in part because the local currency imparts a certain authenticity to your whereabouts (e.g. loonies in Canada, kangaroo dollars in Australia, and Eva Perón pesos in Argentina). Take another look at the coin I used for scale. Just whose faces are those emblazoned on the not-quite-round coin? Why, it’s our very own King William (GVLIELMVS) & Queen Mary (MARIA) on a weathered threepence from the days of the Glorious Revolution in the late 17th century. The coin is 18 mm in diameter (roughly the size of a dime) and a gift from my coin-collecting father.

It’s nice (or is that gneiss?) to be old and good to see William & Mary out and about! Happy Charter Day.