Oncolite is a limestone made of oncoids, the roundish, tan things in the picture (average size less than an inch). Oncoids are made by microbes called cyanobacteria. Cyanobacteria, which also form larger mounds called stromatolites, are thought by many scientists to be one of the earliest forms of life to evolve on Earth.
The microbes attach to a nucleus – in this case fossil fragments – and encrust it in layers of calcium carbonate. The bacteria gather energy by photosynthesis and, thus, require access to the sun. Because they are easy to recognize and mostly limited to shallow marine environments, oncolites are useful to geologists, both as a stratigraphic marker and as an indicator of the depositional environment of the rock they are preserved in.
These were photographed in the Kessler Limestone Member of the Bloyd Formation, northwest Arkansas.
Office of the State Geologist Blog 