The Bechar Basin of northwest Algeria formed as layers of sedimentary rocks from the Paleozoic Era (250-540 million years ago) folded and cracked during collisions of Africa and Europe during the Tertiary Period (2-65 million years ago). Hydrocarbon (coal, oil) reservoirs are tucked among the fragments of sedimentary rocks and fossilized coral reefs that make up the basin's rock layers, which are up to 8,000 meters thick. In this photograph of part of the basin captured by astronauts on the International Space Station on June 27, 2007, dark brown to tan folded ridges of these Paleozoic sedimentary layers extend across the image from top to bottom. Sand dunes are visible to the north, south, and west of the city of Bechar (gray-blue region to the left of the ridges) at image center. (river channels) are dry most of the year in the arid climate of the region. Unconsolidated (loose) sands left in the channels by intermittent streams are transported by surface winds after the water is gone. This leads to the formation of individual dunes and larger dune fields (both bright tan in color) along the wadi courses, which also concentrate sands from other sources; dune fields are visible to the south of Bechar and at image lower right. The oblique view (looking at an angle, not straight down, from the International Space Station) of this astronaut photograph accentuates cliff and dune shadows, providing a sense of the topography of the region.