Dominica's rugged surface is marked by its volcanic past. Rock formations are mainly volcanic andesite and rhyolite, with fallen boulders and sharp-edged protrusions peppering slope bases. The light- to dark-hued clay and sandy soils, derived from the rocks and decomposed vegetation, are generally fertile and porous. Only a few interior valleys and coastal strips are flat enough for soil accumulations of consequence, however. Although scores of mostly mild seismic shocks were recorded in 1986, volcanic eruptions ceased thousands of years ago. Sulfuric springs and steam vents, largely concentrated in the central and southern parts of the island, remain active, however. One of the largest springs, Boiling Lake, is located in the national park.