On the road…again!!!
Essays, Stories, Adventures, Dreams
Chronicles of a Footloose Forester
By Dick Pellek

 The Geology/Archaeology of Angkor Wat

Much of the foundation and structure of the walls at the magnificent religious complexes of Angkor Wat and nearby Angkor Thom in Cambodia were created from building stones that were cut from the soil nearby. Whereas most people assume that rocks and boulders were preformed eons ago at the sites where they are presently found, one soil type known as plinthite, when exposed to the air and sun, dries irreversibly to become rock-like. When used in the construction and framing of the World Heritage Sites at Angkor Wat, the dried plinthite was as fresh as harvested vegetables from the garden.  That is to say, when it was buried beneath the surface of some local soil profiles in the vicinity of Siem Reap, Cambodia, the soil material cum rock that some refer to as honeycomb rock or laterite actually came from a unique type of soft soil that could be cut into blocks.  Plinthite that was exposed naturally through erosion is also referred to as laterite soil. In both cases, whether extracted or exposed, after the full oxidation effects of sun and air, it hardened—irreversibly hardened--into another physical form. In quasi-technical terms, the effects of oxidation on the surface changed the chemical composition and physical properties of the exposed soils, by altering their sesquioxide ratios of iron and aluminum in their soil micelles.

If you go to visit Angkor Wat today, you will notice dozens of honeycomb rocks lying around at various places.  Those blocks were likely “harvested” from plinthite extraction areas nearby and mobilized for the construction of or repair of specific sectors of the complex itself.  One broad area of dozens of honeycomb blocks, in particular, can be thought of as an outdoor workshop.  In the first photo below, the camera was attempting to capture the overall scene as it might have existed even hundreds of years ago when the work was abandoned.  A few stone block working areas are much larger and have also been photographed. 

 Open-air stone workshop

Honeycomb stone (from plinthite soil) repair blocks, upper right 

Chemically speaking, when plinthite alters into laterite, there are changes in the sesquioxide ratios of iron and aluminum, and in the strengthening of the chemical bonds within the soil micelles.  Whereas the soft sub-surface plinthite can be deformed like wet potter’s clay, the resulting laterite cannot be deformed in the same matter.  Although it can be re-wetted under intense rainfall, laterite retains its shape. Thus, the temples of Angkor Wat have remained intact for nearly a thousand years, built from materials taken from the ground…a notable legacy of the land.
 
Other areas of the world have plinthite soils and the craftsmen of those regions also excavate them to use in the construction of foundations, walls, and homes.  The Footloose Forester observed a plinthite quarrying operation on the eastern slopes of Mt. Kilimanjaro in Tanzania, replete with the block drying phase; and has witnessed active plinthite extraction in Viet Nam, only a hundred miles or so downstream from Angkor Wat.  The work site also displayed several blocks that were cut from a hillside and were currently being air dried.