I’m not feeling at my best today (I’ve been laid low over the last couple of days with a mild fever), but I thought that that a brief discussion of Friday’s puzzle was in order. As most of you correctly guessed, the photo was taken in a river channel, and many of the boulders in the photo show signs of being imbricated, which is a fabric generated by the effects of strong currents on flattened or disc-shaped clasts, with large flat surfaces. When such clasts are pointing into a current, then the water will strongly push on the flat face and flip them over so that they point downstream. After this, the current just pushes them down into the stream bed; so over time you tend to get all the flattened clasts pointing stacked on top of each other and pointing downstream.
