Valle de El Golfo Nature · Frontera, El Hierro, Santa Cruz de Tenerife A large natural amphitheatre formed by the collapse of the island's northern slope, and not by a crater, which today holds El Hierro's most fertile lowlands. The Risco de Jinama marks the geographic boundary between the valley and the upper plateau. Photo: barraquito (CC BY-SA 2.0) What to see and do in Frontera