Weathering and biological threats
Many motifs have been destroyed by natural weathering, with frost and water causing severe erosion of carvings on softer sandstone. Water (especially acidic rainwater) is one of the biggest problems for rock art, especially where it pools on the surface, or trickles across it. Vegetation such as moss holds moisture at the rock surface, and lichens make the surface more friable. Roots can be very powerful, breaking stones apart. Dead leaves and pine needles rotting on the surface can also be a problem, changing the chemistry at the rock surface and holding moisture.