Weathering in the Wyborg rapakivi area, southeastern Finland

Abstract

Field survey of weathering sites in the Wyborg rapakivi area indicates that saprolites are far more common in the central part of the massif than in the marginal areas. Many till‑covered saprolites and parautochthonous crust‑tills prove the weathering to be of preglacial origin. The rapakivi saprolites are of grus type. The weathering has caused both mechanical and chemical altera­tion of the parent rock. The clay mineral percentage of the saprolites is low and the formed minerals do not indicate any strong chemical weathering. The most chemically affected minerals are the dark ones and the plagioclases. The residual topography comprises small tor‑like features. Their morphology is guided by the fracturing of the rock