lag deposit and erosion
if you assume the beads are toitally immune to erosion, that their sparse distribution within the white matrix is equivalent everyy\where to that seen in Opportunity Crater, and that the white material weathers to fine dust that blows away entirely...
The Lag deposit represents the concentration of ~10m of eroded rock for every 3 mm of granules. (~ 3x uncertainty, so 3-30m) [This based on the spacing of the beads in any random section]
if the granules are 3 cm thick, they represent 30-300m of eroded rock.