Passage: Surveying started in 1946, and construction began in 1971. By August 1982, less than 700 metres of the tunnel remained to be excavated. First contact between the two sides was in 1983.The Tsugaru Strait has eastern and western necks, both approximately 20 km (12 mi) across. Initial surveys undertaken in 1946 indicated that the eastern neck was up to 200 metres (656 feet) deep with volcanic geology. The western neck had a maximum depth of 140 metres (459 feet) and geology consisting mostly of sedimentary rocks of the Neogene period. The western neck was selected, with its conditions considered favourable for tunnelling.The geology of the undersea portion of the tunnel consists of volcanic rock, pyroclastic rock, and sedimentary rock of the Neogene period. The area is folded into a nearly vertical syncline, which means that the youngest rock is in the centre of the strait and encountered last. Divided roughly into thirds, the Honshū side consists of volcanic rocks (andesite, basalt etc.); the Hokkaido side consists of sedimentary rocks (Tertiary period tuff, mudstone, etc.); and the centre portion consists of Kuromatsunai strata (Tertiary period sand-like mudstone). Igneous intrusions and faults caused crushing of the rock and complicated the tunnelling procedures.Initial geological investigation occurred from 1946 to 1963, which involved drilling the sea-bed, sonic surveys, submarine boring, observations using a mini-submarine, and seismic and magnetic surveys. To establish a greater understanding, a horizontal pilot boring was undertaken along the line of the service and main tunnels. Tunnelling occurred simultaneously from the northern end and the southern. The dry land portions were tackled with traditional mountain tunnelling techniques, with a single main tunnel. However, for the 23.3-kilometre undersea portion, three bores were excavated with increasing diameters respectively: an initial pilot tunnel, a service tunnel, and finally the main tunnel. The service tunnel was periodically connected to the main...

Question: Which rock in the Western Neck is seen at the end?
youngest rock