National Geographic : 1933 Jun
(o) National Geographic Society THE GREAT WALL OF CHINA AT THE NANKOW PASS This, the innermost part of the Great Wall, is well preserved. Its earth core is faced with brick. The outer rampart is20miles beyond Kalgan on the edge of the Mongolian plateau. The Nankow Pass, being one of the principal passes from Mongolia toChina, isheavily fortified. The Wall at Kalgan was the outer line of fortifications; the Wall at Nankow, the second line of defense; and theWalls ofPeiping (Peking), the third and last line.