In stony deserts, when rocks are exposed to atmospheric conditions, they undergo weathering. The cavernous (honeycomb) weathering pattern is one of the most common, but it is still unclear exactly how it is formed. We show that microorganisms, which differ from the surrounding soil and dust, form biological crusts on exposed rock surfaces. These microbes secrete polymeric substances that mitigate weathering by reducing evaporation rates and, consequently, salt transport rates through the rock.