The growth of the city is influenced by the geography of the city, most notably the Toronto ravine system and the Greenbelt, a permanently protected area of green space, farmland, forests, wetlands, and watersheds within the Golden Horseshoe. The natural geography of the city also provided builders with a variety of resources to build from. The most abundant raw material was the shale layer underlying the city, as well as the abundance of clay, making brick an especially cheap and available material, and resulting in many of the city's buildings being built from brick.