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Limestone used in construction deteriorates because the environment to which it is exposed differs from its original sedimentary environment. The differing conditions of moisture, temperature, and pressure change the rock into a more stable state, though this alteration is often relatively destructive. Understanding rock weathering is vital for the fields of architectural conservation and landscape modification. The analysis of small-scale architectural features and patterns can be a useful tool in understanding larger-scale weathering.

The Great Temple of Amman is located approximately 120 km east of the Mediterranean Sea in a semi-arid region with annual precipitation mean ranges from 300 to 500 mm, at an elevation of 838 m. It is located atop the Citadel of Amman (Jebel al Qal’a)-a prominent hill near the urban center (31 degrees 57’N, 35 degrees 53’E). The Roman temple is a large, east-west-aligned colonnaded building atop a stepped podium built of pink limestone ca. 165 A.D. The colonnade, entablature, and architrave collapsed in the great Levant earthquake of 748 A.D., but the podium/plinth course was unaffected and has since remained essentially exposed to the atmosphere. The plinth course was selected for study since it exhibits varied weathering rates ranging from major surface recession to nearly unaffected surfaces displaying original Roman stone-dressing grooves.

Using archaeometric methods, a false datum was created representing the original Roman dressed surface from which the weathering-induced surface recession was measured across two large plinth blocks (southern aspect of 163 degrees N). These Roman dressing marks were used to establish the hypothetical original surface from which weathering has caused the surface to recede. Two blocks of the plinth course were chosen for their identical microclimatic characteristics (i.e., aspect, elevation, lack of obstructions), their varied surface conditions, and original Roman dressing marks indicating the least-recessed (weathered) surface.

Statistical analysis of climatic and lithologic variables with recession depths indicates that the increasingly significant factors accelerating weathering at the site were (1) moisture availability on horizontal surfaces (0 degrees to 20 degrees), (2) insolation on vertical surfaces (90 degrees to 70 degrees), and (3) constituency of mollusc shell material in the limestone. Mean surface recession depths were hive to three times greater on surfaces where mollusc constituency exceeded 50%. Weathering recesses on the plinth course ranged from approximate to 0 to 15 mm +/- 1 mm per millenium, with mean rates of approximate to 1 to 2 mm per millennium for low mollusc content (<10%) and from approximate to 3 to 5 mm per millennium for limestone containing a large mollusc constituency (>50%).