15 September 2023
On a birding hike yesterday morning, I found this:
This is a little slab of kinked phyllite of the Candler Formation. This metamorphic rock started as mud (and ash) deposited atop the Catoctin Formation, and it was later squeezed and heated during Appalachian mountain-building, encouraging the growth of micas at the expense of clay. A strong differential stress made sure those micas grew in one preferred orientation, and that resulted in a foliation. Later, that foliation was squeezed from a new direction, and because it has a strong mechanical layering, it deformed under the new stress by kinking. Held at an angle to the (diffuse) sunlight, the different orientations to the foliation in the kinked and not-kinked parts of the sample reflect the light differently. This creates the appearance of light/dark stripes running through the sample in a still photograph, but those are all the same stuff, just in different orientations relative to the incoming light.
Where have you seen folded rocks lately?