NASA's Earth Observatory has tracked down images resembling all 26 letters of the English alphabet using only NASA satellite imagery and astronaut photography. In this image, the letter 'Y' is for yardangs, elongated landforms sculpted by erosion and similar to sand dunes, but instead comprised of sandstone or siltstone. On December 25, 2000, the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite captured this false-color image of the Ugab River running through what appears to be a field of yardangs in northern Namibia.