The Intermediate Depression, or Central Plain and Longitudinal Valley as it is sometimes called, lies between the Andes and the Coastal mountain ranges and has been caused by tectonic movement that has resulted in subsidence of land blocks. The valley formed is known as a garben and it constitutes a plain that almost runs through the entire country and in some latitudes it can exceed 40 km in width. In contrast, there are also narrow stretches where the Andean and coastal mountain ranges meet, enclosing tectonic basins such as those of Santiago and Rancagua. The Intermediate Depression is intersected by several fast-flowing Andean rivers which during the dry season can have little or no water, thus providing habitats that allow the presence of ephemeral native and exotic plants. The main habitat is of grassland with scattered, mainly spiny trees and shrubs; the dominant species can be Acacia caven, which is now widely thought to represent a historic introduction from Argentina. The exotic weed flora is especially abundant, most of which hails from Europe. Currently, 90% of the Central Plain steppe is impacted by agricultural activities and forestry. Ironically, as with many countries, most of the productive agricultural land is covered by urban expansion.