Page 72 - The Restless Earth Fossils

Finding and excavating Fossils 71

Spotting Fossils

at 65 Miles Per Hour

Paleontologist Kirk Johnson of the Denver Museum of Nature and
Science has learned to spot dinosaur tracks while driving across U.S.
western highways. He describes the process in Cruisin’ the Fossil
Freeway, an entertaining book illustrated by artist Ray Troll. “What
I do is read road cuts the way you read billboards,” says Johnson.
Roads often cut through geological strata like a knife slices through
layered birthday cake. In the Mesozoic Morrison Formation, layers
of sandstone often alternate with layers of mudstone. Dinosaurs
left tracks in mud that later filled with sand. In the Morrison, the
mudstones tend to be deep shades of red and greenish gray while
sandstones appear buckskin tan.
Imagine a sparrow running across the top of a frosted birthday
cake. The sparrow would leave three-toed tracks in the frosting much
like those of its theropod dinosaur ancestors. Each toe would leave a
depression like a small valley in the frosting. If someone covered up
the sparrow tracks with white frosting, that frosting would fill up all
the toe valleys with white. Should someone cut down through those
layers in the middle of a footprint, they would notice that the white
frosting (just like the Morrison sandstone) dips down almost like three
teeth side by side. “So tracks and trackways,” Johnson says, “appear
as bumps projecting off the bottom of the (sandstone) layers.”
Sandstone is harder than mudstone and often “sticks out” in a road
cut where the mudstone above and below has eroded away. When a
sandstone slab finally breaks off, the “bumps” often wind up facedown
toward the ground, meaning that anyone who flips the rock over will see
a footprint cast. Of course, before the sandstone falls, Johnson knows
the footprints will be found on the underside of a sandstone overhang.
He impresses many amateur fossil hunters by finding new tracks and
trackways with this technique, drawn from experience, knowledge of
geology, and the ability to think in the third dimension.

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