A pebble is a clast of rock with a particle size of 4 to 64 millimeters base on the Krumbein phi scale of sedimentology. It is generally described to be larger than gravel and smaller than cobble. A rock made predominantly of pebbles is termed a conglomerate. Pebble tools are among the earliest known man-made artifacts, dating from the Palaeolithic period of human history. They have been found at Olduvai Gorge (1.85 million years ago) and at Longuopo in China (1.8 to 1.5 MYA).