The Collaborative International Dictionary of English v.0.48:

Sarsen \Sar"sen\, n. [Etymol. uncertain; perhaps for saracen
   stone, i.e., a heathen or pagan stone or monument.]
   One of the large sandstone blocks scattered over the English
   chalk downs; -- called also sarsen stone, and Druid
   stone. [Eng.]
   [1913 Webster]