Dark-gray, orange- to brown- to gray-weathering, medium- to fine-grained intrusive igneous rock, commonly fractured into well-developed columnar joints, compositionally similar to basalt; occurs as dikes and sills in both the lowlands and uplands; locally known as traprock. Dolerite is quarried for use as crushed stone.
(geology): An igneous rock with fine grains, consisting of equal quantities of the minerals feldspar and pyroxene. During the Jurassic Period, enormous quantities of dolerite were injected into the Karoo Sequence deposits as vertical dykes and horizontal sills. Because of its resistance to erosion, dolerite dykes and sills have shaped many landscapes, especially the eastern Karoo.