In order to count this Earthcache as
a find, you must complete the following tasks and email the answers
to me.
- Describe the color of the Niobrara
Formation.
- Estimate the height of the
Formation.
- Describe the formation of the
cliffs.
This Earthcache is located between
the manmade Lewis and Clark Lake and the meandering Missouri
River--near Gavin’s Point Dam. The view from this scenic
overlook offers spectacular views of the Niobrara Chalk formations
that line the southern border of the Lewis and Clark Lake.
Enjoy!
The most distinctive bedrock
formation in Yankton County is the Niobrara Formation, also known
as Niobrara Chalk. Chalk is made of a mineral calcium carbonate
that is formed into the shells of microscopic, one-celled,
golden-brown algae called as chrysophyceae. Chrysophyceae are
photosynthesizing organisms. This means that they use sunlight to
make food from carbon dioxide and water. These organisms float in
large numbers at the surface of the sea, gather sunlight for
energy, and scavenge calcium dissolved in water to build their
protective shells. When these organisms die, their skeletons drift
slowly downward--along with their calcium shells. This results in a
slow, soft, invisible rain of tiny flecks of calcium carbonate that
very, very slowly build up at the bottom of seas. The Niobrara
Formation in this area is between 180 and 200 feet thick, but can
be as thick as 600 feet in places along the ridge spanning from
Canada to Kansas.
These tiny flecks of calcium
carbonate built up for tens of millions of years, creating the
Niobrara Formation. This geological chalk formation extends from
Canada to Kansas and dates to 82-87 million years ago, near the end
of the Mesozoic Era (age of the dinosaurs) and deep into the Late
Cretaceous Era (“chalk” period ending with the
extinction of the dinosaurs).
You may be thinking to yourself,
“South Dakota is in the middle of the continent and not even
close to any ocean or seas.” Geologists have concluded that
much of central passage of North America was under water during the
age of the dinosaurs. During the Mesozoic Era, the world was warm
and the oceans were at a high level. The entire central part of
North America was a great, shallow, inland sea known as the Western
Interior Seaway (also known as the Niobraran Sea). This seaway was
created as two tectonic plates collided, causing the Rocky
Mountains to form and creating a depression in the middle of the
North American continent. This large depression and the high sea
levels at the time allowed waters from the Arctic Ocean and the
Gulf of Mexico to flood the central lowlands, creating a sea that
grew and receded during the Cretaceous Era.
North America, at this point in
time, looked a bit like this:
Microorganisms, such as
chrysophyceae lived and died in this sea, slowly accumulating into
deep, dense chalk beds on the bottom of the sea. Mostly extinct
fish, nautiloids, bottom-dwelling mollusks, echinoderms , and
features ending in -saurus also lived in this sea. Fossils of these
creatures have been and still can be found throughout the Niobrara
Formation.
The formation was exposed when the
sea retreated and was covered again with debris by the latest
glacial episode. Following the glacial period, erosion worked to
expose much of the Niobrara Formation. In southeastern South
Dakota, weathering and glacial erosion have completely removed the
Niobrara Formation except for three areas in Yankton County: Lewis
and Clark Lake, Turkey Creek, and Marindahl Lake.
NOT A LOGGING REQUIREMENT: Feel free
to post pictures of your group at the area or the area itself - I
love looking at the pictures.