Angkor, Cambodia: Chapter 3, Petrology

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August 21, 2017

The actual stones at Angkor Wat and the surrounding temple and city complex tell their own, spellbinding story. It’s a tale that can only be read if you’re prepared to do a little digging (both literal and metaphorical).

If you do (and I did), the process of learning about the chemical and geological properties of the sandstones used at Angkor becomes a journey of discovery in its own right. It was a journey that took me to out-of-the way parts of the medieval megacity, looking closely at the pumice-like red laterite.

It also took me to the Ècole Francaise d’Extrême Orient (France’s School of Far Eastern Studies), in its Siem Reap campus, where the library turned out to be a godsend and delayed my departure for almost a week. The EFEO just oozes the French Academy, from its neatly arranged leather-bound editions of EFEO bulletins from the 1930s to the goateed, ruffle-haired academicians vanilla-vaping on the verandah (French academicians, you understand, so none of yer fusty, leather-patched, dandruffy British professors here; they were all spiffingly dressed).

I was most fortunate on this journey to happen across La Corse, the friendly and super-helpful Corsican who suggested using the EFEO’s library. She’s a regular at Tipsy Turtles, where she and her boyfriend favor the chicken cordon bleu because it reminds them of their home cuisine. La Corse has been working with the conservators and archaeologists at the Angkor site. She’d even completed a database of every known Angkor-period temple (thanks for that, mademoiselle) and was a trove of good information.

Detail of one of the structures of the 11th-century Prasats Suor Prat, South Kleang, Angkor Thom, Angkor. Here you can clearly see the Angkorian methodology. Coarse-grained laterite, which has the appearance of pumice (more on that later) forms the bulk of the structure, while finer-grained grey sandstones (used all over the site) are affixed as facing. It’s certain that the whole of the exterior would have been covered in this finer stone: The Angkorians were nothing if not perfectionists when it came to the appearance of their most revered structures. The entire city with its million inhabitants, as well as the royal palace itself, was constructed of wood. Only the buildings related to Angkorian religious worship were of stone. All were temples or sites of devotion save Angkor Wat itself (believed to be a mausoleum for Jayavarman VII’s ashes) and Ta Phrom (a monastery). See below for an explanation as to why the reddish laterite looks so funny, with loads of holes. PIC: JC 2017.

The Information

Quality of information is vital. I found one or two decent and comprehensive web guides, but many were poor quality. Some claimed erroneously that the bas reliefs at Angkor are carved from granite. None had the granular detail I was seeking (‘scuse the pun). For that, I’d have to dive into the pool of papers at EFEO. I’d discover Delvert (1999) and get up with Uchida (1999[i], 2003[ii], 2013). I’d even read Watsantachad (2001)[iii] in an attempt to get my head around how Uchida and others worked out the orientation of the bedding planes from which the temples’ laterite blocks were originally cut in the quarries of Phnom Kulen [iv]. It wouldn’t be a cake walk. I was entering the sliver-thin sub-field of petrology that is not concerned with finding hydrocarbons (oil, gas, etc.).

The material would be good, but not extensive. Geologists had always come to Angkor, of course, but only relatively recently have they gained some understanding of the sources of Angkorian stone, identifying a number of quarries (Fr. carrières) and connecting them with particular periods of building. The French geologist Jean Delvert was working at Angkor in the 1960s, Uchida has been coming since the turn of the millennium, and Watsantachad was looking at the Angkorian temple buildings at Phi Mai in Thailand’s Issaan Province in 2000. I’d also scan Kučera (2008)[v] and others.

It seemed essential to cut through all the fluff and get to the hard-core dope. I also used La Corse’s 100-per-cent reliable file containing a list of every one of the 96 temples included in the World Heritage Site listing, along with the date of construction, stone typology, and state of preservation. Saved me hours of research and cross-referencing.[vi]

This is laterite. Porous laterite, to be precise. Here at the South Kleang of Prasat Suor Prat, the laterite was found by Uchida et al. (1999) to be of the same median pore size as that at Angkor Wat and other temples built in the 11th Century and 12th Century. These high-pore-sized ‘Group D’ laterites were also used at Banteay Kdei and for the towers of Angkor Thom. Looks like pumice, doesn’t it? In fact the holes, which extend through and into the rock, are made by the process of erosion: the washing out of kaolin (essentially, softer clay). What you’re left with is a building material that’s certainly less dense than solid blocks of stone, though its high iron content (the reason for its hue) gives it some heft. This kind of laterite is also very durable as long as it’s covered and doesn’t become a pretty plant holder, as it has here. The chemical composition varies somewhat because laterite is an aggregate of bonded grains of a smorgasbord of stone typology. Primarily containing Fe2O3 and SiO3, (around 35 and 40 per cent respectively), they also contain granites and other detritic material: loose fragments of rock which have been weathered from elsewhere. Angkor’s warm, humid, well-watered environment was ideal for the formation of this type of rock, which is easy to cut and shape. Vast beds of it form much of the Kulen Hills to the northeast of Angkor, from whence the rock was brought to various sites by barge. It’s tough stuff, though not as durable as the roseate sandstones at Banteay Srei (see my piece Stone Seductress for more on the latter). In the roseate sandstones at Banteay Srei, Delvert[vii] found the chemical composition to be high in silica with between 84 and 88 per cent SiO2, certainly a lot harder than many of the grey sandstones used elsewhere in the site, which had varying figures for silica in the 60–70 percentile range. PIC: JC 2017.

The laterites that formed the bulk of the great structures all over Angkor were wholly unsuited for carving. Beautiful though they are in themselves (shot through as they are with thousands of holes weathered into lovely shapes), they’re not what you want as the face of your temple buildings. Sandstones would have to be found for that.

This gorgeous apsara is in the grey sandstone that coats Bayon (13th Century). It’s finer-grained, much more homogeneous, and more durable than the laterites. Apsaras were released during the ‘great churning of the ocean of milk,’ an episode in Hindu mythology. Beautiful name for a mythical happening, but I’m not covering the Hindu backstory here. Since they had so many real live apsaras on hand, the Khmers were reluctant to forgo any opportunity to carve these beautiful figures. Apsaras are Hindu creatures, not Buddhist, and they shouldn’t really have featured in their oeuvre once the empire converted to Buddhism, perhaps a century before Bayon was constructed. The artists could never resist these lovely subjects, however. This sandstone has a relatively smooth finish, finer than that used on the Terrace of the Elephants, but coarser than the grey-green sandstones used for the famed bas reliefs at Angkor Wat itself. Indeed, identifying which precise rock type authors are referring to is not all that simple. Saurin, and early visitor, named what are now called ‘grès gris’ (‘grey sandstones’ in French) by the term ‘grès verts’ (‘green sandstones’) because when samples of the stone are examined under a microscope, there is a lot of green material cementing the grains together. Here, they are largely grains of quartz. PIC: JC 2017.

Beach

The Khmers found their sandstones at Phnom Kulen, about 40 kilometers or so northeast of the main Angkor site, and close to the site of the Khmer’s original great capital on the Kulen plateau. (Yes, there was a capital before Angkor, a huge one constructed of wood. It was called Mahendraparvata (the mountain of Great Indra) and was founded by King Jayavarman II, who proclaimed independence from Java in 804 CE.) Essentially a huge block of varying sorts of sandstone forming a flat-topped range of hills with an average height of 400 meters, Phnom Kulen was home to the Khmers for a century or so, and may never have been fully vacated.

Redder and finer-grained again. This sandstone figure on the northern arm of the Terrace of the Elephants allows for finer work again (compared to the grey stone in the previous picture). The detailing on the headdress is much finer. This busty apsara and the one pictured above are of roughly similar dimensions. The Terrace of the Elephants, built in the twelfth century about 100 years before Bayon, has carving that spanks anything you can see on the famous temple of faces. The composition of what Uchida (1999) refers to as ‘yellowish brown sandstone’ is high in iron and relatively low in Silica oxides. PIC: JC 2017.

The earliest temples at Angkor are tenth-century, so we could speculate that it was not long before the Khmers felt confident enough to come down out of the hills and reap the riches of the plains below. Their control of water (which I’ll cover elsewhere) and abundant rice and fish harvests meant their civilization would flourish for another four hundred years. If you can afford to have thousands of dancing girls hanging about the place (and the Angkorian royals certainly did), you’ve a rich civilization indeed.

[i] Uchida, Etsuo et al. (1999). The laterites of the Angkor monuments, Cambodia. The grouping of the monuments on the basis of the laterites. Journal of Mineralogy, Petrology and Economic Geology. 94. 162-175.

[ii] Uchida, E., Cunin, O., Shimoda, I., Suda, C., Nakagawa, T., 2003. The construction process of the Angkor monuments elucidated by the magnetic susceptibility of sandstone. Archaeometry 45, 221e232.

[iii] Watsantachad, Nuanlak (2001). An Investigation of Sandstone Consolidation Method for the Northern Gopura of the Phimai Sanctuary, a Khmer Monument in Thailand. (Master’s thesis). University of Pennsylvania, Philadelphia, PA.

[iv] On my return to Phnom Penh I called in on my Lebanese friend The Technician at the Dirty Old Sailors. It turned out he’s a trained geologist, and we spent an hour talking of laterites and bedding planes. TT doodled most illuminating diagrams for me on the pad he uses to take orders for beer and food. Thanks TT!

[v] Kucera, J., Novák, J.K., Kranda, K., Poncar, J., Krausová, I., Soukal, L., Cunin, O., Lang, M., 2008. INAA and petrological study of sandstones from the Angkor monuments. J. Radioanal. Nucl. Chem. 278, 229e306.

[vi] My thanks are also due to BT, whose tireless company I enjoyed during my three-day

[vii] Delvert, J., 1963. Recherches sur l’érosion des grès des monuments d’Angkor. :Bulletin de l’École Française d’Extrême-Orient, vol. 51, pp. 453, 534.