to form on

Webster's Dictionary (GCIDE)

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]