Walking to my car the other day, I looked up at the embankment on my street, and noticed some geology there I hadn't seen before. Yesterday, with my camera, I climbed up the embankment (~15 feet) to investigate. Fortunately there were some trees to hold onto.

Sure enough, it was as I suspected: dikes of granite (subvertical in orientation) that, along with the schistose bedrock they cut across, had totally weathered to saprolite.

Keys for scale:


Originally, these dikes were emplaced during the late-Ordovician eastern-North American episode of mountain-building called the Taconian ("Taconic") Orogeny. Later, when they got exposed at the surface (or close to it) they began to "rot."

Hand for scale:


Here's a video showing how readily these dikes formerly known as granite deform by crumbling into pieces:


The main chemical weathering process that has happened here to make this possible is the hydrolysis of feldspar to produce kaolinite, a clay mineral. Large single crystals of potassium feldspar in the granite are now large amorphous masses of kaolinite, which has no strength when stressed.

Labels: dc, granite, igneous, metamorphism, ordovician, weathering