Cretaceous fossil badlands rich in ammonites.
Karai is a stretch of bare, grey clay hills near the village of Karai-Kulakkalnattam, eroded into rounded mounds and gullies with almost nothing growing on them. Geologists call this kind of raw, plant-free terrain “badlands.” What makes these worth protecting is what keeps washing out of them: ammonites, the coiled shells of squid-like animals that lived more than 90 million years ago, along with belemnites, nautiloids, clams and oysters. It is the reference site, or “type area,” for the rock layer named after it, the Karai Formation.
Why is a dry inland hill full of sea creatures? To answer that you have to rewind to when India was pulling away from the rest of the supercontinent Gondwana. In the early Cretaceous, as India split from Madagascar and Antarctica, the crust here stretched and sagged into a long trough called the Cauvery basin. Around 113 million years ago the sea flooded across that sinking floor for the first time, and the land at Karai went under a shallow sea.
On that seabed, fine mud settled slowly, layer on layer. Animals that died in the water sank into it and were buried before they could rot away. The mud was rich in glauconite, a green mineral that forms on slow, oxygen-poor seafloors, and in gypsum, and over time it hardened into the grey-green mudstone and marl exposed today. Ammonites drifted and hunted in the open water; when they died their shells settled into that mud and were locked in.
Later the sea drew back, the rocks were lifted above sea level, and rain took over. The mudstone is soft and clay-heavy, so instead of building soil that holds plants, it washes away quickly into the bare mounds and channels of a badland. Every downpour strips a little more off the surface, and the harder fossils, which resist erosion better than the mud around them, are left standing proud and eventually fall free.
So the badlands and the fossils are two sides of one fact. The same soft marine mud that erodes into a bare wasteland is what buried and preserved the ammonites in the first place, and the erosion that makes the place look lifeless is exactly what keeps uncovering fresh fossils. That is why the Geological Survey of India has declared Karai a National Geological Monument: it is both a clear record of a Cretaceous sea and a site that renews its own collection with every rain.
Sources
- Geological Survey of India — National Geological Monuments
- Early Albian ammonites from the Karai Formation, Cauvery Basin, south India (2019)
- Major geologic events of the Cauvery Basin, India and their correlation with global signatures (2017), Geoscience Frontiers