Ice, dykes and tectonics: the Plattsburgh story

Deborah Painter (USA)

As our small passenger jet began its descent into the Plattsburgh, New York International Airport on a cool November day, I admired Lake Champlain to the east from my window and noticed that the small aircraft, once it touched the very long runway, continued rolling down it for ten whole minutes. When the jet came at long last to the gate area, I noted that the size of the attractive terminal was small – quite out of proportion for that enormous runway.

On returning to the terminal and dropping off my rental car two days later, following completion of an environmental compliance project, I noticed that the young lady who checked my bags and took my ticket at the gate was the same person who loaded the plane’s baggage compartment. Why should such a tiny airport with such a tiny staff and only a few arrivals and departures daily need such a long runway and taxiway? Later, I learned that this had been an Air Force base in the past and the runway had been intended to serve as an alternate runway for NASA’s Space Shuttle in case of an aborted mission.

Less than three years later, I had the good fortune to have another project in Plattsburgh. This time, I took a passenger train and, since the rail line runs parallel to and very close to Lake Champlain through much of its service through New York north of Albany, I was able to see the lake from its headwaters near Ticonderoga to its widest expanse along its New York shores near Willsboro, then on to Plattsburgh.

The northern Lake Champlain Valley area of Plattsburgh and Keeseville, New York has been important to humans for a long time due to its geographic advantage as a lower relief area connecting the St Lawrence and Hudson Valleys (Fig. 1). Beginning as a battleground during the American War of Independence, then serving as a clandestine corridor for American slaves escaping to Canada (the North Star Underground Railroad) and continuing its military focus with a US Air Force Strategic Base with a runway long enough for the Space Shuttle, the area is now mostly a holiday spot, light industrial area and college town.

Some of the earliest American geologists worked here to untangle its complex geology and created the stratigraphic nomenclature for the Lower Palaeozoic Era in North America, particularly the Ordovician System, based in part on the rocks here. In the Chazy Limestone of Clinton County north of Plattsburgh was discovered the very first New York fossil named by science, the Ordovician gastropod, Maclurites magnus.

One such early geologist was Ebenezer Emmons, who is renowned for his work as a young geologist, conducting the first ever geological field trip taken by students at a North American college. During this 1835 trip, he and his Williams College students made accurate observations of the probable palaeoclimatic conditions responsible for the formation of the Carboniferous fossil trees of Joggins, Nova Scotia. This inspired William Dawson to expand on his work and make the site famous the world over. In regard to New York and Vermont studies, Emmons is the founder of American Palaeozoic stratigraphy. He discovered the Taconic system, a segment that extends from Canada down through New York to Alabama. Also known as “Emmons’ Line”, the Taconic system is an overthrust that places Lower Cambrian rocks in contact with Middle Ordovician rocks. The older rocks are the Cambrian Potsdam Sandstone, which Emmons named.

During his time, geologists did not know of plate tectonics and the processes that resulted in this overthrust, now understood to have resulted from the collision of the ancestral North American plate’s leading edge and a plate that was the floor of the Iapetus Ocean. Fellow geologist, James Hall, disputed his assertions about older rocks overthrust atop younger ones, and actually sued Emmons in court over them. Emmons left New York, but continued his teaching and research to further the understanding of North Carolina’s geology.

The Lake Champlain Valley is located between the metamorphosed Precambrian rocks of the Adirondack Mountains west of it and the lesser metamorphosed, but severely compressed, Cambrian period strata of Vermont’s Green Mountain Anticlinorium just east of it. The valley is within a geosyncline in which a prism of sands, marls, muds and silts was deposited at the leading edge of a rising land mass. The entire valley is criss- crossed by numerous thrust faults and normal faults and prone to minor earthquakes to this day. Hundreds of feet of exotic terranes (Emmons’ Line) originally laid down tens of kilometres to the east, were moved.

The sediments became sandstones, siltstones, dolostones and shales. As the millions of years passed, the valley experienced some volcanism, seen today as dykes within older rock strata. Following a quiet period in its geological history, the valley became a drain for the meltwaters of the continental glacier of the Wisconsin glaciation. Lake Champlain (Fig. 2) is what remains of Glacial Lake Vermont. The geological predecessor of the St Lawrence River was Glacial Lake Candona, which held meltwaters from much of eastern Canada. These lakes were temporary in geological terms. An ice dam at the area of modern-day Warwick in Quebec failed approximately 12,000 years ago and Glacial Lake Candona ceased to be a lake.

Lake Champlain’s Ice Age predecessor was estimated to have been a maximum of 270m in depth and extended into Quebec, Canada and possibly as far south as Albany, New York. Lake Champlain’s present day depth does not exceed 121m and its length is 201km in total. It is also an important source of drinking water for the region.

Much of the city of Plattsburgh and its suburbs is low relief and dotted with wetlands (Fig. 3).

Here and there, one can see Quaternary period (Pleistocene epoch) ground moraine glacial deposits of pebbles, tiny grains of rock smaller than sand grains, small boulders and other forms of ’till’, a term for unsorted materials carried and deposited by glaciers. It is interesting to see small boulders, cobbles and gravel scraped from modern day Canada and carried here by the continental glacier (Fig. 4). The geology of the area becomes perhaps more exciting at Ausable Chasm, a tourist attraction since 1870. The first known person to explore it was Major John Howe, who in 1760, used ropes to access it.

Ausable Chasm, just 31.5km south of Plattsburgh by way of Interstate 87 south, is a gorge carved into faults within horizontal strata of Potsdam Sandstone by meltwaters of the Wisconsin glaciation (Fig. 5).

The gorge itself extends for 3.2km along the Ausable River, which drains into Lake Champlain. One can see shallower areas of the gorge by driving along roadways in Keeseville, but to see the best of it, one must take the tours. The deeper portions of the gorge (Fig. 6) are privately owned and furnish opportunities for camping in the owners’ private campground, guided rock climbing, a raft ride and plenty of hiking, as one will have to walk one kilometre just to hike to the raft ride.

In the busy summer season, one might have to wait in line for an hour just to buy tickets. If you do not want to ride in a raft, you can hike some areas of the Chasm, as long as there are no thunderstorms. Many narrow stairs and pathways must be negotiated and one should not wear or carry anything that will get ruined if wet. Because of the rugged topography (Fig. 7), the tours into the gorge are not recommended for very young children, who need to be transported in strollers. However, backpacks for toddlers are available for rent and one can place valuables in a locker at the visitor’s centre.

The return to the visitor’s centre is by shuttle bus, something that is surely appreciated by tired explorers. There are lantern tours and even winter tours, since the chasm is open for visitors year-round. Across the car park for the Ausable Chasm attraction is the North Star Underground Railroad Museum, with educational exhibits and videos on slaves and their experiences getting help on their journeys to upstate New York and Canada, where they could live without the shackles of slavery. Both attractions are located at Route 634 at its junction with Route 373.

About the author

Deborah Painter is an ecologist and general environmental scientist. She lives in the United States.

References

Ausable Chasm Adventures in the Adirondacks web site: https://www.ausablechasm.com/

Falcon-Lang, H. J. September 1, 2009. New Brunswick and Nova Scotia: the First Geological Field Trip by a North American College. Atlantic Geoscience.

Digitally Published: September 1 2009: https://journals.lib.unb.ca/index.php/ag/article/view/atlgeol.2009.005

Friedman, Gerald. 2006. Ebenezer Emmons (1799-1863). Founder of American Paleozoic Stratigraphy: Hero of the Taconic Controversy, one of the Most Celebrated Geological Disputes in North America. Earth Sciences History (2006) 25 (2); 225-238.

North Country Underground Railroad Historical Association. 2022. North Star Underground Railroad Museum: https://northcountryundergroundrailroad.com/museum.php

United States Geological Survey: https://ngmdb.usgs.gov/mapview/?center=-73.827,44.76&zoom=10

Van Diver, Bradford B. 1988. Roadside Geology of New York. Mountain Press Publishing Company. 397 pages.

Woods Hole Oceanographic Institution. December 20, 2004.

Catastrophic Flooding from Ancient Lake May Have Triggered Cold Period: https://www.whoi.edu/press-room/news-release/catastrophic-flooding-from-ancient-lake-may-have-triggered-cold-period/