the country that forms the northern boundary of the United States is Canada :)
Pennsylvania.
pennsylvania and delaware
New York covers much of it except for a 51 mile opening to Lake Erie.
Maryland's northern boundary is defined by the Mason-Dixon Line, which serves as the border between Maryland and Pennsylvania.
This is the boundary with Ukraine: mountains and the river Tisa.
Lake Erie
The sea that forms the northern boundary of Africa is the Mediterranean Sea. The countries of Morocco, Algeria, Tunisia, Libya and Egypt border it.
To the north of Pennsylvania, the state of New York borders its northern boundary. Additionally, Lake Erie forms part of Pennsylvania's northwestern border, while the southern shores of Lake Ontario are also within proximity to the northern part of Pennsylvania.
The western and northern boundary of Pennsylvania extends the furthest west.
The Arctic Ocean
red river