were forests buried?
forests and trees provide direct evidence of the vegetation in Illinois
at a specific time. Ice age sediments sometimes buried trees and even entire
forests. There were several ways this could happen.
tell us a great deal about the environment at the close of the last ice
age. An example of this happened in eastern Illinois at the lower reaches
of a glacier:
ice overrode forests and buried them with glacial
oe is pronounced like the oo in book), wind-blown
silt, coming primarily from the Mississippi and Illinois River valleys,
buried forests over a long period of time.
ice also dammed lakes, causing lake levels to rise and flood surrounding
forests. Rapid build-up of sediments in these dammed lakes resulted in
the burial and preservation of trees in place without any other damage.
Trees remained standing as they had been 18,000 to 24,000 years ago.
ice advanced into Illinois about 26,000 years ago.
open-canopied spruce forest with an understory
of terrestrial mosses developed near present-day Charleston, Illinois,
just prior to 24,000 years ago.
ice reached its southernmost extent only a few miles south and west of
Charleston at Shelbyville, just after 24,000 years ago.
- A lake formed in front of the advancing glacier (proglacial lake), possibly as the result of a damming of the local drainage areas by glacial sediments.
- The lake flooded the forest and killed the trees, which were rapidly buried in the lake sediments.
- The advancing glacial ice then overrode the buried forest depositing glacial drift.
do we know?
that the water that killed the forest was not released in a torrential
flood because of the evidence:
and thermokarst lakes
were not knocked over as they are during torrential floods. Rather, they
remained upright where they grew, rooted in an old soil that had developed
before the last glacial advance in central Illinois. Some of the stumps
are over 2 meters tall with diameters of 12 to 25 cm.
that was deposited was laid down in layers typical of still water. The
layers could represent the differences in sediment from year to year or
from season to season.
Forest beds also frequently occur at the base of lake sediments. These forest beds or 'trash layers' contain a record of forest vegetation that occurred before the lake developed. In Illinois, these trash layers occur in kettle and thermokarst lakes.
lakes form when a block of buried ice left by the retreating glacier melts
and forms a hole, sometimes quite deep. Thermokarst lakes develop when
permafrost (permanently frozen ground), melts locally and forms a shallow
growing on the surface before the ice block or permafrost melted fell into
the newly created lake and formed the trash layer that was later buried
by lake sediments.
Lake in northeastern Illinois (map) has a thick trash layer, which formed
about 16,200 years ago. Scientists from the Illinois State Museum and Illinois
State Geological Survey are now studying this deposit.
from the Illinois State Museum have also studied the trash layer in a thermokarst
lake near Tonica, Illinois. A Museum archeologist discovered this lake while doing survey work before the construction of Interstate 39. It was in a farmer's field and had been drained. The lake formed about 17,000 years ago. As the permafrost gradually melted, spruce trees and other forest plants fell into the expanding lake.
of a now-extinct
stag-moose (Cervalce scotti) was also discovered near the shore of the former lake. The skeleton showed evidence of chewing by wolves, which may have killed the animal in winter on the frozen lake surface. The bones then fell into the lake when the ice melted.
spruce logs at Tonica.
of the deposits can be determined by radiocarbon
dates on wood, cones, or needles from the spruce trees.
at Charleston, for example, dated at 24,000 years old. A spruce log from
Tonica dated at approximately 17,000 years old.
Pollen sequences, macrofossils (leaves, snail shells, insects, bones of other small animals) preserved in buried lake sediments, and the buried spruce trees at Tonica and other sites tell us about the paleoenvironment (paleo = old) in Central Illinois just after the last glaciation.