A couple days ago, I showed a photo of plumose structure here, a feature that sometimes forms when rocks fracture (i.e. a joint is formed). I invoked the image below to show the relationship between the plumose structures and the concentric "ribs" that sometimes show up on a joint (here labeled as "arrest lines"). The point was to show how they were mututally perpendicular.

But the diagram shows something else, too: that the delicate topography of the plumes becomes more exaggerated away from the main surface of the joint, and they grow into twisted "hackles" along the edge of the joint. Joints have ruffled edges! These hackle fringes can also be spotted on many rock surfaces, if you're looking for them.

Here's a photo I took a couple of weeks ago, in the Silurian Needmore Formation (exposed in the Massanutten Synclinorium between Waterlick, VA and Seven Fountains, VA). It shows a series of hackle fringes parallel to one another, showing the growth of the fracture surface over time.

Here it is again, with the Photoshop "contrast" dial turned up to 11:

The high-contrast view helps bring the hackles into high-relief, and also illuminates the subtle plumose structure. Looks like this surface formed from the top, down. As I read it, this joint started on the right side of the image and propagated leftwards as time went by.

(The hematite nodule at left is a bonus feature.)

Labels: concretions, joints, structure