You may be thinking of the Antarctic Circle.
The 42nd parallel forms the southern boundary of Idaho. It marks the border between Idaho and Nevada as well as a portion of the border between Idaho and Oregon.
Limerick is not really a coastal county. However the river Shannon flows into the Shannon Estuary, before entering the Atlantic Ocean. Part of the Shannon Esturary forms the northern boundary of county Limerick. See the related maps below.
Spain is the only country to border Portugal to the east.
The Cell Membrane forms the outside boundary that separates the cell from its environment.
A constellation. (Also an asterism does this.)
the country that forms the northern boundary of the United States is Canada :)
The northernmost point of the Antarctic continent is Prime Head at 63° 12' 48" S, 57° 18' 8" W. All land south of 60 degrees S is governed by the Antarctic Treaty. So, you could say that both 63 degrees S and 60 degrees S -- lines of latitude -- form a northern boundary of Antarctica: your choice.
This is the boundary with Ukraine: mountains and the river Tisa.
The sea that forms the northern boundary of Africa is the Mediterranean Sea. The countries of Morocco, Algeria, Tunisia, Libya and Egypt border it.
No. Antarctica is the southern-most continent on earth. Parts of northern Canada are the northern-most land forms on earth.
The Arctic Ocean
red river
Both Chile and Argentina, close to Antarctica.
The sea that forms the northern boundary of Africa is the Mediterranean Sea. The countries of Morocco, Algeria, Tunisia, Libya and Egypt border it.
Most of the northern boundary is formed by the Alps. However, on the Iberian Peninsula, the Pyrenees form another northern border of Southern Europe.
Mediterranean Sea
The river Danube and the South Dobrogea.