INTRODUCTION TO STRATIGRAPHY
Grand Canyon rocks are mostly sedimentary, and sedimentary rocks form layers or
“strata,” one above the other. The study of layered rocks is called stratigraphy.
Sedimentary strata are deposited in horizontal beds. The oldest layers are at the bottom; youngest at top. No cross sections of strata can be seen unless erosion or faulting
(or a highway road cut!) reveals the
“insides” of the layer, as at Grand Canyon.
Each rock layer, or formation, has a name, usually taken from a local geographic feature.
For example, the Bright Angel Shale is named after the Bright Angel Creek in central Grand Canyon.
Sometimes related formations are combined into a group such as the Supai Group.
All Grand Canyon sedimentary layers formed near sea level as most of Arizona sank.
Modern examples of sinking land include Venice, New Orleans, and most of
Holland.
The Grand Canyon’s sedimentary layers are expressed as alternating cliffs and
slopes. This gives the Canyon its stair-step appearance. The slopes are made of
soft and easily eroded rocks: shale, mudstone and weakly cemented sandstone or
siltstone. Cliffs are the more durable rocks, usually well cemented sandstone
and limestone. Igneous and metamorphic rocks of the Inner Gorge are made of
extremely hard rocks. They are easily hard enough to form cliffs, but their
contorted structure and the easy splitting of schist often causes these rocks to
break into steep rugged slopes as well as cliffs.
All the Grand Canyon sedimentary layers formed near sea level
(just above or just below). Any lowering of the land or rising of the sea
bed would cause the sea to come in. This happened many times. Any deepening of the sea would
inundate the land, for example, during warming trends, the polar ice could melt and deepen the sea.
Each time the sea rose or the land sank, the sea encroached and sediments accumulated, creating various
oceanic layers.
On the other hand, each time the sea lowered or
the land rose, the land became exposed and erosion occurred. In some cases,
rivers brought sediment down from higher regions, making terrestrial
sediment layers. Other times
the wind brought desert sands. The final result is the Grand Canyon as we see
it--many layers, some marine and some terrestrial.
Images, art and text copyright © Dave Thayer, 2010
None of this material may be used for commercial purposes
without written permission of the author.