The geology of the city consists of two principal types of rocks of an ancient Basement Complex, a result of a process that took place during the late Pre-Cambrian period about 500 million years ago. The most extensive rock formation consists of pyroxene granulite gneiss that covers more than 50% of the city area, from the northwest to the east and extends in two small bands southward in the western part of the city. Syenitic gneiss accounts for about 40% covering the southern, western and north-eastern corner of the city and also in form of scattered small intrusions into the pyroxene granulite gneiss. In their unweathered state, both rock types are impervious hence poor aquifers, but they provide a valuable resource for road and building construction. Blantyre City is on the eastern edge of the southern branch of the Eastern African Rift, and thus some prominent faults occur within and in the vicinity of the city rendering it prone to earthquakes. The main faults and associated zones of deeply fractured bedrock run from the north of the city in a southwesterly direction and are potential aquifers for groundwater.