Mexico-USA Border and Algodones Dunes
The border between the Mexican state of Baja California and the state of California, USA, runs through the center of this radar (ASAR) image.
On the left, the city of Mexicali, Mexico, appears bright white. Just above is its sister city, Calexico, California. The border appears as a line, between the two cities, accentuated by the difference in organisation and color (intensity of grey) of the crops near them.
The line continuing eastward from the Mexicali area is actually not the border, but rather the US Highway 8 running just north of it.
On the Mexican side of the border, on the bottom left, part of the Sierra Madre Occidental mountain range can be seen in light grey and white.
On the USA side, the dark area is a patch of desert. The diagonal line through it is the Algodones Dunes, a large erg (sand dune field). It is approximately 72 kilometers long by 10 kilometers wide (45 miles by 6 miles) and extends along a northwest-southeast line that correlates to the prevailing northerly and westerly wind directions.