Natalya Polina was showing me around a geological museum in southern Siberia. She explained, “This is a cross-section from a stalactite. The rings show how much it has grown each year”.1 My interest was aroused by a very wide ring which grew well over a centimetre in one year (see picture on the right).

“What causes the rings?” I asked Natalya.

“The dirt that surface water carries into the cave varies between summer and winter,” she replied. “There is more dirt in summer so a dark ring is deposited, whereas in winter, when the surface is covered with ice and snow, the deposit is cleaner.”

An internet site on geology gives a similar interpretation:

“Paleoclimatologists analyze the growth rate of stalactites and stalagmites to reveal patterns of past rainfall. This graph shows the thickness of near-annual growth rings for the past 450 years from a stalagmite in Carlsbad Cavern. Thick rings indicate a relatively wet climate, while thin rings indicate a dry climate.”2

The scale on the graph in question is graduated in 0.1 mm steps, from zero to 0.3 mm. By contrast, the Siberian ring was about 15 millimetres wide—50 times greater than the widest rings on the Carlsbad chart. This is because the Carlsbad Cavern is found in the New Mexico desert, where rainfall is low, even in the wetter years. But the stalactite Natalya Polina showed me was recovered from the Altai Mountains of south-western Siberia, where summer rainfall is much greater….

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