fortran-continuation-string

fortran-continuation-string is a variable defined in `fortran.el'.
Its value is

"$"

• This variable is safe as a file local variable if its value
  satisfies the predicate which is a byte-compiled expression.
  

Documentation:
Single-character string used for Fortran continuation lines.
In fixed format continuation style, this character is inserted in
column 6 by M-x fortran-split-line to begin a continuation line.
Also, if M-x fortran-indent-line finds this at the beginning of a
line, it will convert the line into a continuation line of the
appropriate style. Normally "$".

You can customize this variable.