Thin Section of Crenulation Cleavage

Thin section (cross polars, quartz plate) of crenulation cleavage developed in a previously deformed and schistose metapelite. The trace of the crenulation cleavage (e.g., red) runs from the upper left to the lower right; the preexisting cleavage has been folded (e.g., green). Note how the proportion of quartz (brighter colored grains) to phyllosilicates is much lower in the limbs of the folds than in the hinges. Photo credit: John Ramsay.