Fluid Flow and Jointing

Thin section of folded calcareous phyllite (with crenulation cleavage developed in phyllosilicate-rich layers) cut by a joint plane, Brigels, central Alps, Switzerland. The joint is clearly later than fold formation; no movement can be detected across the joint surface. The pale brown coloration in the joint walls is the result of deposition of limonite (iron hydroxide) from solutions passing along the joint plane. Photo credit: John Ramsay.