The oblong rocks at the Eureka Springs Gardens in Arkansas, USA, bring a curious smile to passing tourists, once they inspect them closely.
Grey and smooth, the rocks have a fabric imprint, resembling coarse canvas sacking. They look remarkably like sacks of flour! The bottom of one sack is elongated and even preserves a pattern of stitching. The top is pulled together, complete with petrified wrinkles, as if it was once tied with rope.
Although they look like bags of flour, they are heavy and difficult to lift. The largest one weighs 38 kg (84 lb)—over three times heavier than a similar amount of flour.
The bags feel different too. A bag of flour is normally soft, and bends when it is lifted. These bags are as hard as rock. They make a hard, sharp sound when slapped, and would break your knuckles, too, if you punched them.
The little sign gives the story: “You are looking at parts of petrified flour sacks from the Blue Spring mill.’ Although not suitable to eat, these sacks of petrified flour give lots of ‘food for thought”.
Blue Spring holds the secret of these petrified flour sacks. It is a tranquil circular pool at least 155 m (510 ft) deep, into which cool water rises silently from the earth at 150 million litres (38 million gallons) per day. It is a feature of the gardens.
Since the 1840s, spring water was used to drive a large mill, and grind wheat and corn (maize). The bags of flour were almost certainly made in the mill, and left abandoned when it stopped operating around 1903.
The bags became petrified after they were saturated by water from the spring. Minerals from the limestone strata dissolved in the spring water before it flowed from the earth. Later those minerals precipitated in the waterlogged flour sacks—turning them into solid rock.
A small sample of petrified flour was chipped from one of the bags for analysis. It was like hammering hard rock. Microscopic examination revealed that the flour was still present, but all the air space had been filled with tiny calcium carbonate crystals. There was no burlap bag remaining—it must have rotted away.
We are told repeatedly that petrification (or petrifaction) requires an unimaginably long time—millions of years—to occur. Because of this cultural conditioning, people are surprised to discover that rocks can form quickly. This sense of surprise is the reason for the exhibit, and the notice alongside the sacks explains it all:
“It is commonly believed that petrification is a process taking millions of years … not true! Under ideal conditions petrification can take place in as few as three weeks.”
It’s a fact. Petrification does not need millions of years. Rocks can harden very quickly under the right conditions. The evidence is all around us. We just need eyes to see and willingness to accept the obvious.