offshore as separate island arcs not yet joined to the continent. The Wrangellia and Alexander terranes merged offshore by the Middle Jurassic and experienced folding and faulting during a collision with Alaska in the Cretaceous. The addition of the terranes generated metamorphism and led to the intrusion of huge amounts of granite in the Coastal Ranges. By the end of the Cretaceous, a fourth landmass—the Chugach terrane—docked with Alaska. Geologists have interpreted the Alexander terrane as a volcanic island chain surrounded by coral atolls, given volcanic rocks, chert and limestone from the Paleozoic. Rocks in Glacier Bay and the Heceta-Tuxekan Islands to the west of Prince of Wales Island preserve clam shells and reef life. In many locations, Alexander terrane limestone rests on top of conglomerate made up of beach boulders. Volcanic rocks, carbonates and phyllite overlie these lower rock units, intruded by Jurassic igneous rocks. Many sections of the terrane are highly metamorphosed and broken into smaller blocks by faults, such as the Chatham Strait Fault, which offsets rocks up to 120 miles. The Stikine terrane includes Paleozoic volcanic rocks interbedded with marine shale, limestone and sandstone, overlain by Jurassic and Cretaceous sedimentary and igneous rocks. Thrust faults and the Chatham Strait Fault separate the small Taku terrane from neighboring terranes. It is marked by metamorphosed andesite, rhyolite and basalt with small amounts of limestone, marble and conglomerate. On top of the Coast Range batholith are remnant metamorphic rocks from the gneiss, schist and marble dominant Tracy Arm terrane. The Wrangellia terrane, as one of the largest in Alaska formed beginning 300 million years ago in the Pennsylvanian and built up with basalt, carbonate and phyllite during the Triassic (these rocks are exposed from the Alaska Range to the Wrangell Mountains). Originating as a volcanic island arc, Wrangellia exhausted its magma supply and began to sink, although renewed volcanism built up basalt and the Nikolai greenstone above the water level, until the new land sank again covered over by limestone and deep water sediments. The Chugach terrane, by contrast, is made up of turbidite deposits from the Mesozoic, along with conglomerate, shale and volcanic rocks. During the Cretaceous, tectonic activity in the Arctic Ocean created a series of sediment filled basins, which now host oil and gas deposits, underlying the continental shelf of northern Alaska. Organic-rich shale overlies a major unconformity in the area, acting as the source rock