A mountain of Jurassic rock
Phu Kradueng National Park is a massive, heart-shaped sandstone mesa that rises 1,000 meters above the surrounding plains. The park's highest point reaches 1,316 meters above sea level. This entire plateau, covering about 60 square kilometers, is a remnant of the Phu Kradung Formation. This geological layer consists of siltstones, shales, and sandstones deposited by rivers and floodplains during the Late Jurassic to earliest Cretaceous period, roughly 145 to 160 million years ago. The sandstone that caps the mountain is more resistant to erosion than the underlying rock, which is why this towering plateau exists today.
The cliffs and rock outcrops across the plateau's surface display a remarkable form of weathering called tafoni, or more specifically, honeycomb weathering. This process creates networks of small, rounded cavities, typically less than two centimeters in size, that resemble the cells of a honeycomb. These features are not carved by insects but by a combination of physical and chemical processes acting over immense timescales.
The science of sculpted stone
The formation of honeycomb weathering at Phu Kradueng is a polygenetic process, meaning multiple factors work together. An important mechanism is salt weathering. Moisture from rain and fog penetrates the porous sandstone, carrying dissolved mineral salts. As the water evaporates from the rock's surface, these salts crystallize within the rock's pore spaces, just beneath the surface. This process, known as subflorescence, generates immense pressure as the crystals grow. The force is strong enough to pry individual sand grains apart, breaking down the cement that holds the rock together.
Cyclic wetting and drying, along with daily temperature fluctuations, accelerate the process. As the rock heats and cools, the mineral grains expand and contract, further weakening the structure. Wind then carries away the loosened sand grains. Small pits begin to form in weaker parts of the rock. Once a small cavity is created, it establishes a microclimate that traps moisture, accelerating weathering inside the pit and causing it to grow. Over thousands of years, adjacent cavities merge, forming the complex, lace-like patterns visible today.