Complex sentence: The border between these two districts is Union Street from Prospect Park West to Sixth Avenue and then President Street from Fourth to Sixth Avenue; north of this border is District 13, south of this border is district 15.
Split into simpler sentences: The border between these two districts is Union Street from Prospect Park West to Sixth Avenue and then President Street from Fourth to Sixth Avenue. North of this border is District 13, south of this border is district 15.