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]

The Collaborative International Dictionary of English v.0.48:

Marl \Marl\, v. t. [imp. & p. p. Marled; p. pr. & vb. n.
   Marling.] [Cf. F. marner. See Marl, n.]
   To overspread or manure with marl; as, to marl a field.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Marl \Marl\, v. t. [See Marline.] (Naut.)
   To cover, as part of a rope, with marline, marking a pecular
   hitch at each turn to prevent unwinding.
   [1913 Webster]

   Marling spike. (Naut.) See under Marline.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Marl \Marl\, n. [OF. marle, F. marne, LL. margila, dim. of L.
   marga marl. Originally a Celtic word, according to Pliny,
   xvii. 7: "Quod genus terrae Galli et Britanni margam vocant."
   [root]274.]
   A mixed earthy substance, consisting of carbonate of lime,
   clay, and sand, in very variable proportions, and accordingly
   designated as calcareous, clayey, or sandy. See Greensand.
   [1913 Webster]

WordNet (r) 3.0 (2006):

marl
    n 1: a loose and crumbling earthy deposit consisting mainly of
         calcite or dolomite; used as a fertilizer for soils
         deficient in lime