The low rounded wall in the foreground surrounds the opening above the stairway down to the stream from the hatchway in the living room. The ceiling above is glass, the only glass ceiling in the house. Open to the stream below and to the sky above, the two are connected by a vertical "column of air." This "column" of openness is on the southeast corner of the living room (and of the house), while the massive fireplace and chimney on the northwest corner of the living room (or, more dramaticaly, the west tower on the northwest corner of the house) represent a vertical column of stone (and in the case of the west tower, open from top to bottom - 3 stories - by a wall of glass). These vertical lines have their counterpart in bold horizontal lines, such as the massive cantilevered levels and large terraces that reach out from them. (See also this view of the house from the waterfall). In the case of the southwest terrace, the only part of the house directly over the falls, the double cantilevering gives an impression of a tree branch growing and then spreading out in different directions horizontally.
One can actually see straight through the south end of the living room
to the southwest terrace through the glass doors on the left in the
photo above. Thus we find here a horizontal counterpart of space -
defined by openness and freedom of flow - to that of the "column of air"
mentioned by Edgar Kaufmann, Jr. (Kaufmann's son, who studied with
Wright) in his beautifully illustrated, large format book, Fallingwater: A Frank Lloyd Wright Country House.