Walking down a trail somewhere on Block Island, I caught sight of a  light-colored dike (vein) that seems to be sharply bent.  This doesn’t make sense because there are no other fractures and no other indications of folding. What you’re looking at is optical illusion created by a pair of planar facets on the surface, each of which is one side of an old fracture.  These facets change your angle of view in such a way that they make the dike appear to be bent.   Proof that this is so is the next photo, taken parallel to the plane of the dike.  From this direction, the angle the facets don’t change the appearance.

This mini-mystery was only one of many delightful things I saw during a trip to Block Island in mid July to run a field trip, give a lecture to the community (sponsored by the Block Island Gardeners), and enjoy the cool fog.  Everyone who was on the trip with me looked at this optical illusion, and found it interesting.  Another was the standard or “archetype” block island wall, taken in the southwestern corner of the island, where the land is preserved from future development.

Note the uniformity of pink granite, the general blockiness of the stone shape, and the subequal mix of semi-rounded and angular fragments.  This indicates “bumper-car” style of boulder traffic in the basal shear zone of the ice sheet, which rounds stones until they break, only to be rounded again.

 

 

---