The Rocks Don't Lie: A Geologist Investigates Noah's Flood Read Online Free PDF Page A

apply to all rock types. 2
    Everywhere on Earth is either eroding and losing material or receiving deposits of material eroded off of somewhere else—one geologic realm sheds sediment, the other accumulates it. But the places where each is happening change over time. The most obvious change apparent in the walls of the Grand Canyon is that the marine rocks exposed in it have switched from one domain (deposition) to the other (erosion). Eroded upland environments are not preserved in the rock record because there’s nothing left to see—they’ve vanished. The geologic signature of mountains is recorded by its absence, a gap in the record of time, while the story of our planet and life on it is archived in the sediments of depositional lowlands and marine environments—the places where sediment piles up over geologic time.
    Deciphering earth history involves establishing the basic relationships between different rock formations and the nature of the boundaries, or contacts, between them. Two layers of sedimentary rock deposited one atop the other without any discontinuity are considered conformable—they accumulated with minimal interruption. An eroded surface leaves a discontinuity between two rock units, a gap representing missing time that geologists call an unconformity. An unconformity represents how far down erosion wore into an ancient landscape before additional sediment was deposited on top. A whole series of unconformities exposed in the canyon walls tell of multiple rounds of deposition, deformation, and erosion before the whole package of rocks rose from the sea to the level at which we find them eroding today.
    After far too many switchbacks, I made my way out of the inner canyon and across a cliff of flat-lying rock. In passing, I traced my finger along the surface of the unconformity where the Tapeats Sandstone rests on the irregular surface of the Vishnu Schist. The now solidified grains of sand settled onto a rocky seabed in the Cambrian Period, about 100 million years before plants began colonizing land.

    The great unconformity at the base of the Grand Canyon where the Tapeats Sandstone truncates the Vishnu Schist ( based on a sketch by Véronique Robigou ).
    The span of time this unconformity represents is staggering. More time lies missing between the 1.7-billion-year-old Vishnu Schist and the 525-million-year-old Tapeats Sandstone than is recorded in the enormous wall of rock soaring thousands of feet overhead. All the lost time was enough to erase a mountain range and hide the ruins of ancient worlds, familiar in design but alien in detail. My imagination wrestled with how a thousand million years could vanish from the geologic record. Two worlds had come and gone, leaving nothing behind but their rocky bones.
    I kept walking up, leaving the unconformity behind, and noticed burrows and tracks of simple wormlike animals in the cliff of Tapeats Sandstone through which the trail ran. The fact that marine life was crawling around the bottom of an ancient sea, as documented in the solid evidence right in front of me, presents a serious problem for the creationist view of the Grand Canyon. How could fragile worms have been crawling around on and burrowing into the seafloor during a flood powerful enough to remodel the planet? The biblical flood would have to have dumped more than ten feet of sediment every day for a whole year in order to have deposited the thousands of feet of sediment exposed in the canyon walls.
    Finally I reached an area known as Indian Gardens, a broad bench a third of the way up the canyon. This scrap of flat ground offered shade and a welcome break from slogging up switchbacks. Trailside exposures of the hard-baked mud of the Bright Angel Shale showed why this relatively flat ground graced the side of the canyon. Rock fragments burst from beneath my boot as I kicked an outcrop in passing. The shale was too weak to hold a cliff.
    I looked closely at the shale and saw more signs of