Vegetable or Mineral?
Recently, my older daughter and I joined a team of about a dozen parents and children taking part in a regional stream cleanup organized by local environmental groups. Our squad took on the stream in the woods, clambering over its tumbled rocks and pulling out refuse--everything from last week's blue grocery bags to a 20-year old license plate. The real prize, however, was not a human-made relic: My neighbor Gary Gillespie wrestled up a rock that looked like a chunk of firewood, with bark intact. "Look at this," he said, plopping the slab onto the path that runs along the stream. "Petrified wood!"
I was skeptical, if only because I'd studied a bit of local geology in order to write a column (Charmed Life, Nov. 15, 2000). Just a short distance up Herring Run, I recalled, the rocks date back a billion years--long before there were such things as trees. Nonetheless, a short distance overland can span a vast stretch of geologic time. Despite my doubts, I was excited by the idea of finding fossils at the bottom of my hill.
Gary's rock--sitting beside me as I write--is strikingly woody in appearance. It's about as long and wide as a shoebox, and half as deep. One side is cylindrically curved, like the exterior of a tree, and irregularly ribbed, much like bark. The other side has a distinct lengthwise grain and a chunky, broken texture, like the interior of a split log that has begun to rot and chip away. I've seen rocks with similar surfaces but never one so convincingly loglike.
Gary kindly indulged my curiosity by letting me cart the specimen to the Maryland Geological Survey (MGS) on St. Paul Street, where geomorphologist Jim Reger hefted it and studied it with a lens. "I've seen samples of petrified wood that didn't look a nickel's worth of difference from this," Reger said, but the sheer density of the rock gave him pause; most petrified wood is much lighter, he explained.
There are a few specimens of petrified and semipetrified plant life on display in the MGS library. The oldest is a tree trunk, 250 million years old. Other samples, half that age, show a remarkable amount of grainy detail. Reger said that some younger pieces--mere tens of millions of years old--are still woody enough to be burned, "although the sulfur smell would drive you away." One such specimen, an ancient cypress log, was turned up during the construction of the Ravens' stadium.
According to Reger's geologic map of Baltimore, the slopes surrounding my little neighborhood stream are built of Loch Raven schist, a brown, grainy rock typically flecked with mica. At roughly half a billion years in age, the schist predates life on dry land. An entirely different kind of rock, called pegmatite, crops up in the streambed. It consists of big crystals, dirty white to rusty orange, and looks nothing like Gary's specimen.
Explaining that rocks per se aren't his speciality (geomorphology focuses more on the features of the planet's surface, such as hills and valleys, and the forces that create them, such as erosion), Reger said he'd show the mystery slab to some colleagues. He also offered to cut the specimen in half, which would expose clues to its true nature. Gary OK'd the surgery, but Reger cautioned me that a petrified log wouldn't necessarily show growth rings inside: Petrification--the replacement of once-living tissue with minerals--can result in rocks that look homogeneous to the naked eye.
I left the rock at MGS and revisited the stream to see if I could find anything else resembling petrified wood. Aside from the native bedrock, the streambed featured an assortment of pebbles washed from upstream, and nonlocal rubble dumped in the name of erosion control. Although I didn't find what I was looking for, I had the pleasure of startling a sizable water snake, which coiled itself on the bank and stared at me.
The next day, back at MGS, I got a peek inside the mystery rock, which the agency's new rock saw had sliced as cleanly as knife-cut fudge. To my untrained eye, the cross section looked almost as woody as the exterior, with bands of color following the curve of the "bark" and fine cracks just like those that naturally occur in logs. Reger's colleagues, however, judged the rock to be some sort of gneiss, akin to the billion-year-old boulders near Morgan State. The streaks inside would result from the natural layering and stretching of minerals; the rippled "bark" could derive from the uneven wearing of harder and softer surfaces. The rock's tiny internal crystals and its density were not consistent with petrified wood.
I can live with ambiguity. Nature produces plenty of pseudo-fossils: crystalline "dendrites" that look like ferns, concretions that mimic living forms. If eons of sedimentation, pressure, and erosion have produced a facsimile of organic growth, I take it as one more example of how fractal geometry rules the material world. Regardless of its composition, I'm fairly convinced that the log-rock didn't originate in the spot where Gary found it. It was probably dumped there as rubble.
Meanwhile, Jim Reger's geologic map assures me that there are fossils yet to be found in the Baltimore soil. Most of downtown and many city neighborhoods are built on clay deposits that date back to the Cretaceous era--prime time for dinosaurs, circa 100 million years ago. Small fragments of partially petrified ferns, conifers, and cycads (palmlike plants) show up in this clay fairly often, and similar clays, lying along the Route 1 corridor between Baltimore and Washington, have yielded dinosaur bones. Are you down there, Baltosaurus?
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