From the smithsonian:

“I tip the torchlight and examine a wall in my hotel room. From a distance, the wall looks like vanilla frosting roughly applied. Up close, I see nuggets of caramel-colored stone, faint brown streaks…and an oyster shell. The wall before me is 100 million years old, the raw edge of a cave scraped into a cliff above the Loire River. The oyster was a much earlier guest here, a fossil left from the sea that once covered this part of France and left behind a thick bed of white stone called tuffeau.

Many buildings in the Loire Valley are constructed from this stone. On a trip to France four years ago, I stayed in an elegantly restored farmhouse near Tours, its walls made of tuffeau blocks, stacked like irregular sugar cubes. The farmers of long ago probably dug their own tuffeau. It’s just under the surface–unplanted fields gleam with tuffeau churned to pebbles by the plows. However, the serious quarrying was for the signature chateaus and other monumental architecture of the Loire Valley.

At the time, I was among friends who wanted to visit all the chateaus. The first few exhausted my taste for opulence. Then, near the chateau in Amboise, I noticed caves in the cliff, some with brightly painted front doors, windows, shutters and flower boxes. As we drove around the Loire Valley, I spied more of these domesticated caves, some with chimneys thrusting through scruffy vegetation at the tops of cliffs or new facades and courtyards. Oh yes, someone finally explained: after widespread quarrying of the tuffeau began in the 11th century and created cavities in the hills and plains, people moved in. Some to escape warfare, others because the caves made convenient, low-rent dwellings. Until the early 20th century, many people lived in these so-called troglodyte homes. Entire villages were underground. Some people still live in the caves, I was told, and others are …”

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