According to inside sources at Caltech, "...it is "Throop Pond" located in the midst of "Throop Memorial Garden" in memory of the first building here on campus "Throop Hall" (destroyed during an earthquake in the 1970's)..." And apparently there's something significant about the way the rocks and and organisms are arranged, something to do with respective ages or some such, but as I didn't pay much attention to the plaque I couldn't really tell you...