Have you ever found, while sifting through the pea gravel on the playground, a perfectly cylindrical pebble with ridges like a screw, and a hole through the center? If you have, it was probably a fossilized crinoid stem, part of a prehistoric marine animal similar to a starfish. The crinoid, also called the "sea lily," used this stem to anchor itself to the bottom of the shallow sea that once covered most of North America. Crinoids were abundant for hundreds of million of years, so crinoid fossils are common. They are appealing objects, distinctive, and fun to hunt. Long before modern paleontology, Native Americans and Europeans threaded crinoid stems onto strings as talismans. Catholics in England called them "St. Cuthbert's beads," and used them in rosaries.
The gravel under the swings and slides of Knoxville's older public playgrounds is full of fossils and shells. For a child crazy about dinosaurs, hunting for crinoid stems on the playground may whet an appetite for amateur paleontology.
Excavating under the monkey bars at Parkridge Park with a few local kids, I find: a handful of crinoid stems, a few fossilized shells, a speckled fragment that was once probably dinosaur skin (we hypothesize), and a disconcerting contemporary artifact: the silver-capped tooth of a child. We also found plenty of broken glass and tiny bits of plastic toys, which may one day, themselves, enter the fossil record. This tendency of pea gravel to accumulate debris is considered a negative property by the U.S. Consumer Products Safety Commission. But pea gravel remains a stimulating educational tool for learning about geology, paleontology, and, occasionally, anthropology.
I had thought these stones on the playgrounds must be leftovers from the 1980s. I was surprised to learn the city of Knoxville still uses pea gravel to maintain older playgrounds. Knoxville follows CPSC guidelines that recommend a safety surface on all playgrounds, and pea gravel is the cheapest playground surfacing material approved by the CPSC. The city also uses wood chips, recycled shredded tires, and a few newer playgrounds have something called a unitary surface—a poured-in-place virgin-rubber mat on a bed of tire "mulch."
The American Society of Testing Engineering tests these materials and rates their safety by dropping an instrumented model of a child's head onto different materials and measuring their shock-absorbing ability. This test yields a "critical fall height," the height below which a child could fall head-first and be expected to survive. Of all the materials used by the city of Knoxville, pea gravel has the lowest-rated effectiveness in cushioning falls, while shredded tires have the highest.
Despite its high ratings, shredded tires may not be the best safety material. When scattered outside the boundaries of the playground, it becomes litter, impossible to pick up, not biodegradable, and containing exotic chemicals. Sometimes, concerned moms call the city asking if the wood chips are treated, or if Round-Up is sprayed on the pea gravel to kill grass. The answer to both those questions is no. But, and this seems to mystify one city official, no one has called yet to question the safety of the shredded tires—a relatively new and unknown product, not originally manufactured to be tasted by toddlers.
The official adds that as tires shouldn't be incinerated, shredding and reusing them is a positive in the consumption/reuse cycle.
Wood chips seem to be the best loose-fill material from an ecological, maintenance, and safety perspective. Playground mulch is made of coarse, untreated hardwood chips—not yard waste—and its ability to cushion falls is almost as good as shredded tires. Wood chips are comfortingly biodegradable, which means they do have to be replenished every three-five years. Kellems Mulch installs the chips by blowing them in with a giant hose from a tractor trailer truck, apparently an impressive sight.
In the 1950s kids in Tyson Park played—and fell—on the unyielding surface of an asphalt playground. Today, the rubber unitary surface of the Tyson Park playground has the highest over-all safety rating. It is highly effective at absorbing shock, has no small pieces to swallow or throw, and is the only wheelchair-accessible surface. Of course, it is also the most expensive.
Still, I believe the ancient river gravel does the most to expand young minds, as long as they don't fall on it head first.