Plenty of visible rock nodules embedded on the surface of the sandstone, but they are not original deposits

In Zion National Park, Utah, USA, Visitors can frequently observe abundant rock nodules embedded on the surface of the sandstone. However, these nodules are not original deposits. Geologists explain that this sandstone was originally a vast network of sand dunes. Following sedimentation and burial by upper layers, the loose sand grains were bound together by iron-rich cementitious solutions to become solid sandstone. Later, groundwater carrying dissolved iron filled the pores within the sandstone matrix, precipitating to form near-spherical, iron-rich nodules. Because these iron concretions are significantly harder than the surrounding sandstone matrix, millions of years of differential weathering have left them exposed as isolated spherical knots on the rock surface.