I'm working on a research paper about him right now. its mind blowing how everything in his houses tie together. this is from one of my sources about Fallingwater
"Symbolically the hatch plays an even bigger role in the house's, orientation, 30 degrees east of south. "If you trace the line from the entrance into the house to the far corner of the living room toward the terrace that overshoots the falls, that diagonal aligns perfectly with the path of the stream below," says Levine. "If you then take a line from the fireplace to the hatch, that line crosses the other diagonal at 90 degrees, and it aligns with the ledge that creates the falls. Typically you mark a site with a cross. In this case, the cross is locked in, aligned with those two very important aspects of the site: the sound and the movement of the water."