Complex sentence: During the 19th century extensive iron resources were found in the nearby hills, this boosted the industrial revolution in the zone and the estuary of Bilbao was covered by steel and shipbuilding industries and the docks expanded from Bilbao to the bay.
Split into simpler sentences: During the 19th century extensive iron ore deposites were discovered in the nearby hills, which boosted the area's industrial base. The estuary of Bilbao was covered by steel and shipbuilding industries and the docks expanded from Bilbao to the bay.