The area is underlain by a series of Tertiary sub-aerial basaltic flows and breccias and rhyolite ash flows of the Masset Formation, which form a plateau volcanic sequence dipping gently to the north.

Tachylyte occurs as a flow-like mass in rhyolite units of the Tartu Facies. Tachylyte is a black, glassy basalt, with a brilliant lustre, speckled with white, rectangular phenocrysts of feldspar.