Individual roots of any plant modify the soil immediately surrounding themselves. The combination of water and humic acids can cause the precipitation of cement, especially calcite, between the soil particles surrounding the root. Erosion of the poorly cemented sediment between the more resistant fossil roots (or rhizoliths) can leave a 'lag' deposit of these fossil roots. The surface shown in the picture above is littered with hundreds of these short fossil root fragments.
In detail, the individual rhizoliths commonly have a sub-millimeter diameter tubular hole down their centers where the actual root was located before rotting or decaying out of the fossil. The outer surface is very irregular being controlled by nearly random fluxuations in the distance of chemical trasport and reaction surrounding an individual root.
Here weathered out rhizoliths litter the ground around the base of a non-symmetrical pillar.