National Fossil Wood Park, Sathanur

Near Sathanur lies a single fossil tree trunk about 18 metres long, a conifer whose wood has turned entirely to stone. It was found in 1940 by the geologist M. S. Krishnan of the Geological Survey of India, and more logs of the same age sit in the ground at villages within about 10 kilometres. The wood dates to the Cretaceous, roughly 100 million years ago (some accounts put it at over 120), long before flowering plants took over the land.

The strange part is that the sea is now about 100 kilometres to the east, yet this tree ended up in marine rock. Sathanur sits in the Cauvery basin, a trough that opened as India rifted away from Madagascar and Antarctica during the breakup of Gondwana. In the Cretaceous the sea reached much further inland than it does today, transgressing westward past Sathanur, so the coast ran close by. Conifers grew on that shore, since conifers were the dominant trees of the age, well before the first flowering plants.

Here is how a tree becomes a rock. A flood swept trunks off the land and into the shallow sea, where they were buried quickly in mud and sand, fast enough to keep out the oxygen and microbes that would normally rot wood away. Groundwater seeping through the buried log carried dissolved silica, the same mineral that makes quartz and glass. Bit by bit the silica settled into the wood and took the place of the original tissue, copying its shape as it went. The result is petrified wood: stone that still holds the outline, and often the grain, of the tree it replaced.

That slow swap is why the log survived at all. Ordinary wood would have decayed in years; silica turned this one into something that could last a hundred million. And because it was buried in sediment laid down under a Cretaceous sea, the log doubles as evidence of where that sea once reached. The Geological Survey of India protects it as a National Geological Monument, a piece of a coastal forest from a world with no flowers, preserved because the sea buried it and groundwater turned it to stone.

A short reference note: this summary draws on third-party sources such as the Geological Survey of India and Wikipedia. In time I plan to give every geological feature its own learning-oriented write-up — with custom diagrams, graphs, and study pointers — as I’ve already done for the 11 locations that have an Instagram reel.

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