sentences:  Known as the Arrowhead for its shape, this region shows the most visible evidence of the state's violent past.There are surface exposures of rocks first formed in volcanic activity some 2,700 mya during construction of the Archaen-Superior province, including Ely greenstone, metamorphosed and highly folded volcanics once thought to be the oldest exposed rock on earth; Proterozoic formations created about 1,900 mya that gave the area most of its mineral riches; and more recent intrusive gabbro and extrusive basalts and rhyolites of the Duluth Complex and North Shore Volcanic Group, created by magma and lava which upwelled and hardened about 1,100 mya during the Midcontinent Rift.The Precambrian bedrock formed by this activity has been eroded but remains at or close to the surface over much of the area.