District History

Leetsdale Archaeology

Lewis and Clark National Council

Organization

Pittsburgh District

Great Lakes Division

USACE

ARCHAEOLOGISTS FIND ANCIENT PEAT BOG UNDER LEETSDALE CASTING BASIN

At the bottom of the Leetsdale casting basin, archaeologists recently found an ancient peat bog. This hole, dug by archaeologists on site, shows the layering profile of the peat bog.

Archaeologists digging on site at the Leetsdale casting facility found an ancient peat bog while digging below the now empty casting basin, where Braddock Dam Segments #1 and #2 were constructed.

This is an exciting find! Scientists have long wondered how long it took for temperatures to rise and the vegetation and climate to change once the glaciers receded from the northern portion of the United States after the last Ice Age ended 10,000 years ago.
Now they are about to find out.

The information that is obtained at this peat bog will be applicable to most regions of the U.S. that were on the glacial margins during the last Ice Age. These ancient peat deposits will be radiocarbon dated to find out how old the material is -- seeds, pine needles, sedges, and other vegetation have been identified at Leetsdale.

In the upper two peat layers, called lenses, a temperate zone beech woodlands appears to have been present. The lower peat lens has heath vegetation. Did it take thousands of years . . . or just 100 years for the climate to change from heath to beech woodland? This will be determined after some samples of the remains are submitted for radiocarbon dates.

How was the peat bog formed?
In the case at Leetsdale, it appears that water swept leaves and other debris down the Ohio River. Here some of that debris ended up in a higher pocket, like a mud puddle. There was enough velocity and soils in the water that a swirling pattern created by those ancient flood waters could be seen by the archaeologists.

Before the debris could decay another flood episode deposited soils over it sealing out the oxygen and bacteria that cause decay. The leaves, etc. became peat (well-preserved organic remains). Then another flood swept leaves and other debris over the same area. This was again sealed by another flooding episode, creating a second peat deposit. This preserved another lens of peat.

What are peat lenses?
Peat layers are referred to as lenses (singular is lens) because they occur in a relatively restricted location. In other words, we wouldn't expect peat to be found everywhere beneath the Leetsdale site. In fact archaeologists excavated an exploratory hole in the summer of 2000 about 50 feet from where this peat deposit was found and there was no indication of peat at that location. The peat was found about 20 cm (8 inches) above cobbles. Cobbles mark the bottom of the Ohio River at the end of the Pleistocene (last Ice Age).

Links:
USGS: The Great Ice Age
Ice Ages