An impressive inselberg rising some 600 m above the surrounding plain and 1600 m above sea level, Gross Brukkaros is the only topographic feature releaving the monotony of the flat country between Mariental and Keetmanshoop, which is part of the Nama-Karoo Basin. It has a basal diameter of ca. 7 km, and with a steep-sided ring-shaped ridge bordering a central depression, has the typical shape of an extinct volcano, but actually resulted from the interaction of hot magma (molten rock) and groundwater known as phreatomagmatism. Brukkaros rests upon flat-lying reddish-brown sandstones and shales of the upper Nama Group (ca. 530 million years), which were overlain by tillites and shales of the Karoo-age Dwyka Group (ca. 220 m.y.) during the Gondwana glacial period. Subsequent uplift of the southern African sub-continent caused most of the Dwyka beds to be removed, but a few remnants which escaped erosion occur locally on the eastern and southwestern slopes of the mountain.