above sea level at Centre Hill, but lacks any well-defined peaks. To the northwest it is bounded by the Caribbean Sea and by the Southern Tobago fault system to the southeast. Rainfall averages about per year. Most streams run along oblique-slip faults with a northwest orientation. Major streams draining the Main Ridge include the Coffee, Hillsborough, Goldsborough, and Queens Rivers. Streams which drain to the northwest are short and steep, while those which drain to the southeast are longer, with better-developed courses. Waterfalls, including the Argylle Waterfall (Tobago's highest) occur at major transitions between rock types. The northeastern slopes are steeper than the southwestern ones and are among the most landslide-prone parts of the island. Geology Main Ridge is primarily underlain by the North Coast Schist Group (NCSG), which occupies the northern third of the island of Tobago. The island of Tobago is the main exposed portion of the Tobago terrane, a fragment of crustal material lying between the Caribbean and South American Plates. The rocks of the NCSG are metavolcanic; the underlying igneous rock was laid down during the Late Jurassic or Early Cretaceous. They underwent metamorphosis prior to the mid Cretaceous, when additional volcanic intrusion formed the rocks of the Tobago Volcanic Group. A combination of uplift, erosion, and faulting during the late Mesozoic and Paleogene led to the elevated horst block separated from the half graben of the southern lowlands by the Southern Tobago fault system. Vegetation and ecology Main Ridge is dominated by lower montane rain forest (according to John Stanley Beard's classification of the vegetation of Tobago). In Tobago, these forests are characterised by an emergent canopy of Licania biglandulosa and Byrsonima spicata, with the palm Euterpe broadwayi more common on exposed ridges and summits. Moist lowland forest dominates the lower-lying portions of the Main Ridge. The forests experienced limited human disturbance prior to the 1940s, but were badly damaged by Hurricane Flora in 1963. Sixteen mammalian species including the nine-banded armadillo, the crab-eating raccoon and the red-rumped agouti, 210 species of birds, 24 snakes and 16 lizards have been recorded from the area. The ichthyofauna is limited - only four of Tobago's 13 freshwater fish species are found in its rivers and streams. The spotted algae-eating goby, Sicydium punctatum, is the most widespread fish species in the area, while the jumping guabine, Anablepsoides hartii, is found at the highest elevations and is the only