Which preposition to use with altmark

in Occurrences 1%

Hieronymus Dieterich Berendis was born at Seehausen in the Altmark in the year 1720, studied law in the University of Halle, and was for some years after his student days auditor of the Royal Prussian Regiment of Hussars, usually called the Black Hussars from their uniform, but at the time named after their Commander von Ruesch.

Which preposition to use with  altmark