Geology Reference
In-Depth Information
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 mil-
lion 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 Sand-
stone 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 mil-
lion years could vanish from the geologic record. Two worlds had come and gone, leaving
nothing behind but their rocky bones.
