The Canterbury Plains formed from the erosion of the Southern Alps (an extensive and high mountain range caused by the meeting of the Indo-Australian and Pacific tectonic plates) and from the alluvial fans created by large braided rivers. These plains reach their widest point where they meet the hilly sub-region of Banks Peninsula. A layer of loess, a rather unstable fine silt deposited by the foehn winds which bluster across the plains, covers the northern and western flanks of the peninsula. The portion of crater rim lying between Lyttelton Harbour and Christchurch city forms the Port Hills. Estimates suggest that native forest once covered 98% of the peninsula. However, Māori and European settlers successively denuded the forest cover and less than 2% remains today, although some reforestation has started. European settlers have planted many English trees, notably walnut. Hinewai Reserve, a private nature reserve, has been established on the peninsula to allow for native forest to regenerate on land that was once farmed. Several sites off the coast of the peninsula serve for mariculture cultivation of mussels. A large Marine Mammal Sanctuary, mainly restricting set-net fishing, surrounds much of the peninsula. This has the principal aim of the conservation of Hector's dolphin, the smallest of all dolphin species. Eco-tourism based around the playful dolphins has now become a significant industry in Akaroa. The relatively small Pohatu Marine Reserve centres on Flea Bay on the south-east side of the peninsula and the larger Akaroa Marine Reserve lies at the entrance to the Akaroa Harbour. The Summit Road forms a notable feature on the peninsula. Built in the 1930s, the road is in two sections (both of which have views of the area, as well as parks, walkways, and other recreational features):  An attraction for trampers and bushwalkers is the Banks Peninsula Track. Following the major earthquakes of 2010 and 2011, which affected Christchurch and Lyttelton (the harbour serving Christchurch), cruise ships were diverted to Akaroa Harbour.