Ptygmatic folding is a style of folding characterized by an "intestine-like" appearance. The folded item (in this case, a granite dike) folds back on itself, and the cross-cut material (in this case, a schist) flows out of the way. In other words, there's a viscosity contrast between the relatively-stiff granite dike and the relatively-weak schist.

This particular ptygmatically-folded dike is in a boulder outside of the National Museum of the American Indian, in Washington, DC. That's a quarter at the top of the photo, for scale.

Labels: dc, igneous, museums, structure