The Winooski River Gorge

The Winooski River Gorge at South Burlington is a gorge that formed when the Champlain Sea began to drain. The original Lake Vermont was a muddy, silty body of water stretching from Quebec possibly down south as far as Albany, New York. Much of silt and sediment settled on the bottom of the lake creating the soil that the Winooski valley is known for.

Formed by the melting Laurentide Ice sheet Lake Vermont emptied suddenly and quickly when an ice dam located near Warwick Quebec gave way. The lake drained roughly 300 feet in a very short span of time. Due to the massive weight of the Laurentide Ice Sheet the present day surface of the Lake Champlain Valley was about 500 lower than it is today. This placed the greater Champlain Valley below sea level and after the Lake drained sea water rushed in creating the brackish Champlain Sea. As the land continued to rise the seashore retreated leaving beaches behind. One of these former beaches now lies on the terrace that forms the Burlington Airport. Eventually the lake level rose above sea level leaving just the present day Lake Champlain basin filled with water. Unable to be replenished by the ocean it gradually lost its salinity.

As the land level rose the Winooski River cut new courses in the sand, silt, clay and mud of the former sea bed that now defined it's delta mouth into the inland sea. In this relatively easy sedimentary material the Winooski river is wider and slower, meandering across the landscape. Some obstructions to this meander do occur. Lying across the path of the Winooski in the region of the Winooski gorge is a layer of Vermont Valley Sequence Bedrock. Comrprised primarily of Shelburne Marble (Lower Ordovican) this bedrock also includes dolostone and gray limestone beds.

Shelburne Marble is light-buff, cream, bluish-white, or mottled. It includes thin beds of light gray dolomite. In the Winooski gorge there is also some evidence of Dunham Dolomite or 'red marble' which is red and white mottled dolomite that was laid down in the Cambrian era, and is among the youngest of the Cambrian dolomites.

Limestone is a sedimentary rock composed principally of calcium carbonate (calcite) or the double carbonate of calcium and magnesium (dolomite). It often includes fossils, shell fragments and other debris.

Here at the Winooski Gorge there is a large deposit of Limestone, and in the past there was a massive limestone quarry here. It even passed under the road in a limestone tunnel, connecting the eastern half of the quarry to the western half.

In this satelite photo you can clearly see where the Winooksi River ran into the obstructing bedrock that constrained the river. Once past the obstruction it was once again able to cut a meandering path through the silt and clay. To recieve credit for this earth cache please send me your thoughts on the following:

1) From the parking lot, if you walk uphill towards Saint Mikes and look back out over the fields (looking upstream) what visual evidence do you have that the river encountered some sort of obstruction here at the Winooski gorge?

2) Examine the sides of the old quarry and the gorge. Based on the visible evidence do you think the ridge of bedrock continues? Why or why not?

3) Looking down into the gorge from the bridge describe the rocks that are here. What color are they? Is there evidence of further erosion? Can you see any Dunham Dolomite in the cliffs?

4) (optional) Include your own picture of the gorge.

Sources:

• https://dec.vermont.gov/geological-survey/publication-gis/VTrock
• https://www.burlingtonfreepress.com/picture-gallery/life/2021/04/29/quarry-below-lime-kiln-road-colchester/7337521002/
• https://ngmdb.usgs.gov/Geolex/UnitRefs/ShelburneRefs_3798.html
• https://soilseries.sc.egov.usda.gov/OSD_Docs/W/WINOOSKI.html
• https://crowspath.org/natural-history/glaciers/glacial-lake-vermont/
• https://dec.vermont.gov/geological-survey/vermont-geology/staterx/Dunham