Stratigraphy: Rock Layering
rocks of the Bryant Watershed are sedimentary.
"Sedimentary" means that particles, like soil or sand, or minerals, have
dropped or crystallized out of the water (or in some places were blown
there by the wind). These sediments then built up in layers and turned
to rock over a long period of time. Here in the watershed you find them
in nearly horizontal layers. You can see this layering exposed along the
bluffs and natural outcrops and most clearly in highway roadcuts (Figure
1). This layering shows that these rocks began as layers, or beds,
of mud on the floor of an ancient seabed. These sedimentary beds vary
from inches to several feet in thickness. The different beds, and the
lines called bedding planes that separate them, record changing conditions
while sand and mud were being deposited on the sea floor. By studying
this layering, geologists learn volumes about the ancient environments
in which the sediments were deposited.
If you observe the layering in the mud at the bottom of
a puddle or a stream, it seems obvious that the layers on the bottom of
the pile were there before the ones on top and are therefore older. This
simple observation has given geologists a means of telling relative time
in the history of the development of the rock layers. Those layers on
the bottom are older than those on top. Therefore, this pile of layered
rocks becomes a record of time - a history of the events that occurred
during the formation of the rocks.
The layering you see in a roadcut is just a small part of
the overall picture. Geologists have grouped these layers into major units
called "formations", which can be hundreds
of feet thick. These formations can be traced or correlated across wide
areas over hundreds of miles. Variations in the formations across a region
tell geologists even more about the ancient environment. Geologists want
to know about these ancient environments to help search for petroleum
and other natural resources.