To form on

from The Collaborative International Dictionary of English v.0.48

Form \Form\, v. i.
   1. To take a form, definite shape, or arrangement; as, the
      infantry should form in column.
      [1913 Webster]

   2. To run to a form, as a hare. --B. Jonson.
      [1913 Webster]

   {To form on} (Mil.), to form a lengthened line with reference
      to (any given object) as a basis.
      [1913 Webster]