Due to the size of the object about the scale of a small car I would theorize that the artifact is either the capstone of a much larger pyramid possibly buried deep beneath the surface or perhaps a marker stone.
Due to its near perfect design and shape that this anomaly is the result of intelligence design and certainly not a trick of light and shadow.
Link Image:
http://mars.nasa.gov/msl/multimedia/raw/?rawid=0978MR0043250040502821E01_DXXX&s=978