Extensive coastal outcrops and moderate relief combine to provide an excellent exposure of the 14-km2 Greenhills Complex. This complex consists of a deformed layered series of rocks, including dunite, wehrlite, and gabbro, of Permian age intruded into Lower Permian metasedimentary rocks of the Southland syncline. Widespread slumping in the layered series and the emplacement of the complex as a plug of hot fluid mush indicate recurring conditions of instability during evolution of the complex. Emplacement of the complex was accompanied by the intrusion of innumerable dikes and by folding and faulting; the material of many of the dikes was derived from the complex. Country rock, dominantly volcanic microbreccia with intercalated bands of impure marble, increases from regional prehnite-pumpellyite facies through greenschist facies to a marginal gabbroic rock, itself the result of contact metamorphism equivalent to pyroxene-hornfels adjacent to the complex.
The layered series of the Greenhills Complex is stratigraphically divisible into: (1) an upper gabbroic portion dominated by an approximately 640-m-thick gabbro zone which includes a 15-m-thick allivalite unit; (2) a lower ultramafic or peridotitic portion comprised of a 150-m-thick wehrlite zone (which includes near its base a poikilitic wehrlite unit less than 30 m thick), and a basal dunite zone exceeding 600 m in thickness. Feldspathic wehrlite occurs in a narrow transition zone between the upper and lower portions of the complex. The layered series shows well-developed accumulate structures and textures resembling cumulates in classic stratiform intrusions. Cryptic layering is shown by the range in composition of the primary minerals. These minerals in order of separation from the Greenhills magma are: olivine (Fo90–65), clinopyroxene (Ca41Mg54Fe5 to Ca44Mg47Fe9), and plagioclase (An92–88).
The Greenhills Complex is essentially the result of crystal settling during fractional crystallization of a basaltic magma, modified by fluid mush flow and mixing of rocks along mutual contact zones. The magma is believed to have had affinities transitional between alkaline and tholeiitic magma types. Differentiation took place in a hydrous open system connected with the surface of the earth.