In one corner of the Early Life hall of Smithsonian's National Museum of Natural History stand two petrified tree trunks. They are 207 million years old, stony relics of an ancient conifer (Araucarioxylon arizonicum) that once flourished in prehistoric Arizona. Smithsonian curators refer to them as the "Sherman Logs," because they were collected in 1879 at the instigation of Gen. William Tecumseh Sherman, the man who led the March from Atlanta to the sea near the end of the Civil War.
Back in the Triassic period they flourished in what is now known as the Black Forest, part of the 93,492-acre Petrified Forest National Park in Arizona. Prehistoric Arizona was a flat stretch of tropical turf in the northwest corner of a supercontinent known to modern geologists as Pangaea.
The trees towered as high as 200 feet and measured more than 2 feet in diameter. Once felled, they "turned to stone" almost instantly (at least in geological terms, meaning anywhere from tens to hundreds of years). Partly because of the fossil evidence they contain, we know what felled them. Arizona could be hot back in the Triassic period, and it had wet and dry seasons on a grand scale--in keeping with the earth dramatics of the time. During the wet season, megamonsoons swept across Pangaea and turned dry-season streamlets into thunderous, roiling rivers.
One rainy season more than 200 million years ago, an enormous volcano added to the confusion by blowing its stack and overloading the already overflowing rivers with ash and debris. The waters jumped their banks and fanned out in a Ganges-like flood, charging across the floodplain, and toppling the big trees, burying them at least 30 to 40 feet deep. Shut off from air, and thus protected from oxidation, bacteria and fungi, the trees did not rot; the wood was instead gradually replaced by rock-hard minerals like the glassy silica so abundant in volcanic ash.
These minerals began to crystallize between the waterlogged wood's cell walls. As the cell structure itself broke down, crystals formed in those spaces as well. In some logs iron oxides and other minerals provide a rainbow of colors (but logs that still contain some organic material, like the Sherman Logs and most others from the Black Forest, tend to be dark).
After about 20 million years or so Pangaea separated into a northern landmass, known as Laurasia, which eventually split into Eurasia and North America, and a southern landmass, known to geologists as Gondwanaland, which evolved into South America, Africa, Australia and Antarctica. Meanwhile, encased in sediment, the Sherman Logs were alternately covered by a shallow tropical sea or salted by huge sand dunes. Sometime during the logs' interment, geologic uplift, erosion and grumbling earthquakes snapped the brittle tree trunks into surprisingly neat sections. In a process that is still going on, the seasonal downpours that once buried the logs began to expose them, by washing away the earth in which they were entombed.
Time passed. The dinosaurs waxed and waned. Ice ages came and went. Primates began to come into their own--and started collecting things. Thirty-five thousand years ago dawn-age naturalists--Neanderthals, in fact--began arranging fossils and lumps of iron pyrite in orderly rows in a cave--perhaps the first museum. In A.D. 40, Caligula, a tough and often bloody-minded Roman emperor, supposedly ordered his troops to collect seashells for posterity--the "spoils of the sea" on the shores of Gaul. And in the autumn of 1878 another commander, the restless General Sherman, spotted "extraordinary specimens" of "petrification" while traversing the new Arizona Territory. From his headquarters in Washington, D.C., Sherman, who was also a regent of the new Smithsonian Institution, wrote to Col. Peter T. Swaine at Fort Wingate, New Mexico, telling him to collect some "interesting samples" of "petrifications for the Smithsonian."
The general told the colonel exactly what he wanted. "I would like to secure at as little cost of money as possible two logs, say from 20 to 25 inches diameter and six or eight feet long, weighing near a ton each. . . ." Sherman envisioned two logs that "would stand on end, in either side of the Museum. . . . Therefore," he noted, "they should match somewhat in size" and should "be encased with wood for transportation" and addressed to the Smithsonian "on the easiest terms possible, for the Smithsonian is poor in money, though rich in the treasures of knowledge."
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