On Cambrian and Ordovician sandstone inselbergs at Al-Quwayra in southern Jordan, many near-vertical rock walls exhibit marked case hardening. Detailed examination of the case-hardened layer from four sites on both red and white sandstones has been carried out using Schmidt hammer, simple chemical analyses, scanning electron microscopy, energy dispersive spectroscopy and inductively coupled plasma mass spectrometry. These analyses reveal a range of types of case hardening, often in association with cryptoendolithic bioﬁlms containing cyanobacteria and fungi. Iron, manganese and calcium are all found to be common constituents of cements within the case-hardened layer. Micro-organic bioﬁlms may assist in the formation of case hardening through aiding cementation, but are by no means a necessary component of all the hardened layers in the Wadi Rum area. Copyright © 2004 John Wiley & Sons, Ltd.