“Inside‐out weathering” of boulders in glacial outwash gravel

Abstract

Abstract Some small boulders and large cobbles of indurated quartzofeldspathic sandstones in glacial outwash gravel in North Westland, New Zealand, have weathered cores and radial cracking widening to the surface. They are found sporadically in gravel pits and road cuttings up to more than 12 m below the ground surface in a region that has been uplifted and deeply dissected. The cracking is attributed to slight expansion of the core probably mainly as a result of the hydration of very fine grained secondary mica and the development of hydrated iron oxide by weathering. Initially the clasts will have been saturated below the water table, but when this dropped during uplift and dissection, the exteriors dried rapidly but the interiors retained water long enough to result in mineral alteration.