The Collaborative International Dictionary of English v.0.48:

Greensand \Green"sand`\ (-s[a^]nd`), n. (Geol.)
   A variety of sandstone, usually imperfectly consolidated,
   consisting largely of glauconite, a silicate of iron and
   potash of a green color, mixed with sand and a trace of
   phosphate of lime.
   [1913 Webster]

   Note: [hand]Greensand is often called marl, because it is a
         useful fertilizer. The greensand beds of the American
         Cretaceous belong mostly to the Upper Cretaceous.
         [1913 Webster]

WordNet (r) 3.0 (2006):

greensand
    n 1: an olive-green sandstone containing glauconite