A Gap in the Geologic Record
The black basalt that caps the cliffs before you
stands in stark contrast to the colorful Chinle
Formation visible throughout the Painted Desert.
Below this layer of basalt, a horizontal line cuts
across the face of the mesa and separates rocks of
two different geologic periods. The pink
mudstone below this line belongs to the Chinle
Formation deposited about 225 million years ago.
The brown mudstone and basalt layers above the
line represent the Bidahochi Formation deposited
only 5 to 8 million years ago.
How is it that only this thin line represents more
than 200 million years of geologic history? This
gap is known as an "unconformity." Geologists
believe the missing layers were more than 1,000
feet (305 m) thick and that they were eroded over
the years by runing water and wind—forces that
continue to shape this landscape. The
emplacement of the basalt has temporarily
stopped erosiion on the Painted Desert rim, while
it has continued in the valley below.
Cycle of Erosion and Deposition
The 225-million-year-old Chinle
Formation was buried under
younger rock layers
Erosion cut away younger rock layers
exposing the Chinle Formation.
Starting eight million years ago,
lakes and volcanoes reburied the
Chinle Formation under the much
younger Bidahochi Formation.
Recent erosion has removed most
of the Bidahochi, re-exposing the
The upper black layer is basalt that flowed from
a volcano that erupted here. Below the lava an
"unconformity" (see arrows) indicates missing
layers, or a gap in the geologic record.
Geologic map of Petrified Forest National Park
Sand dunes and stream deposits of
the Quaternary Period (present to
one million years ago)
Basalt and sedimentary outcrops of
the Bidahochi Formation (5-8
million years ago)
Mudstone and sandstone of the
Triassic Chinle Formation (208-225
million years ago)