Opportunity Rover found that the soil at Meridiani Planum was very similar to the soil at Gusev crater and Ares Vallis; however in many places at Meridiani the soil was covered with round, hard, gray spherules that were named "blueberries."  These blueberries were found to be composed almost entirely of the mineral hematite.  It was decided that the spectra signal spotted from orbit by Mars Odyssey was produced by these spherules.  After further study it was decided that the blueberries were concretions formed in the ground by water.  Over time, these concretions weathered from what was overlying rock, and then became concentrated on the surface as a lag deposit.  The concentration of spherules in bedrock could have produced the observed blueberry covering from the weathering of as little as one meter of rock.  Most of the soil consisted of olivine basalt sands that did not come from the local rocks.  The sand may have been transported from somewhere else.