The Ozark Plateaus and the Ouachita Province are actually two physiographic provinces, related spatially and compositionally. Both regions are composed of Paleozoic sedimentary rocks. To the north, the Ozark Plateaus are formed by an asymmetrical low dome, steep on the eastern slope and shallow on the west. The rocks of the Ouachita province have been deformed and eroded to form a series of east-west trending ridges and valleys. The boundary between these provinces is marked by the Boston Mountains, a dissected plateau in the southern Ozark Plateaus; and the Arkansas Valley, a vast lowland in the north of the Ouachita Province.