The north facing cave of Choukoutien, designated as Upper Cave, was discovered when removing the regolith of Choukoutien Locality 1 on the top of the hill in order to locate the southern extension of the Sinanthropus beds. The cave was filled with a different kind of deposit—gray, slightly consolidated, fossiliferous breccia. The loose gray sediments were in contact with the reddish, hard Sinanthropus beds and were disconformable and interbedded by a thick stalactite floor. The bones contained therein looked much less fossilized than those generally found in the Choukoutien Formations. It was thus supposed that the deposits represented a very young and rather uninteresting formation, so no further investigation was carried out at that time. The site was just mentioned in “A Note of the Physical Characters of the Prehistoric Sinanthropus Pekinensis in China” in Memoirs of the Geological Survey of China. A systematic excavation on the site began on May 20, 1933; and the fossil finds were then discovered to be much richer and more revealing than what had been anticipated.