Wednesday, July 31, 2019

Varvey Interesting

     I see a book. A 13,000 year old book! Looking at the same scene, most people would see nothing more than a bare, muddy bank. And they'd probably put a little distance between themselves and the nut case babbling on about a book. But look closely. See those thin horizontal layers stair-stepping up the slope. Don't they resemble the pages of a big, open book lying flat on a table?

     These are clay varves. They are another of the many landscape artifacts bequeathed by glaciation. Drumlins, eskers, deltas - those are big topographic features difficult to miss. Varves, on the other hand, lay mostly hidden and it takes some kind of recent excavation to expose them. Even then, they are often visible for just a short time before being cloaked by vegetation...books once again stored in the basement where no one sees or reads them.

     Varves are annual layers of sediment deposited in post-glacial lakes. They occur in light/dark couplets. The lighter layers are rock flour released during the summer melt. They tend to be slightly coarser - silt and fine sand. The thinner, darker layers are clays that settled out in the still waters of winter. Varves vary in thickness depending on the warmth of the year in which they were deposited. They are somewhat similar to tree rings in that respect, building a record of the climate when they were created. Geologists use this fact to construct chronologies of glacial retreat.

Varve core showing winter/summer deposits 
- from North American Glacial Varve Project

Matching varve cores from different sites
- from North American Glacial Varve Project

     Much of the early research on varves was done by Gerard De Geer in Sweden. Indeed, 'varve' comes from the Swedish word for layers. Also in the early years of the last century, Ernst Antevs was using New England varves to document the recession of the ice front. His methods, refined and built upon over time, are still used to date deposits.

De Geer second from left, Antevs on right
- from North American  Glacial Varve Project

     While examining my varved slope I came across odd, rounded inclusions that seem to be the precursors of concretions. They appear to be of the same clay as the varves but slightly harder so that they remain when the surrounding material is eroded away. The concretions added strength comes from being cemented by percolating ground water containing silica, carbonate or iron oxides. There were reddish stains near the ones I saw suggesting iron oxides as the cementing agent. 

     Look for varves (and concretions) in the lowland areas of Washington County. These places were under the waters of Lake Albany and Lake Vermont and that's where the clays and silts settled out. Sometimes road work and ditching will expose them. Streams that have incised into the sediments can also reveal varved banks. 

     My photos come from a spot where the town highway crew had graded a bank that was slumping into the road. It's in a Hudson soil near what would have been the western shore of Lake Albany. The elevation is approximately 185 feet above sea level. I estimate the shale bedrock to be some 25 to 50 feet below the exposed varve and nearby field surfaces are 30 to 40 feet above. There could potentially be as much as 100 feet of varved clay deposits at this location although only about ten feet can be seen.

     Finally, here's one last varve tidbit. When I was a kid my buddies and I used to play on what we called the clay slide. It was a steep bank maybe 25 feet high that dropped into the Snook Kill stream. This was way before the time of water parks and Moms who shuttled their kids from one activity to the next. We were poor, our parents were always working, and we made our own fun out in the wild. I remember hauling buckets of water to the top of the bank. Poured down the slope, this made the clay slick enough for a heart-thumping descent and crazy splash into the water at the bottom. But it wasn't all smooth sliding. More like bumpity, bump, bump all the way down - no doubt because of the varved layers in the clay. Eventually we wore thru the bottoms of so many pairs of cut-off jeans that our mothers made us stop. To this day I think I still have bruises on my butt from the clay slide. But don't panic. There will be no photo documentation of that on this blog. 


     * A great site for more information is the Tufts University North American Glacial Varve Project. Access it here.

Varves from Lake Vermont
- from North American Glacial Varve Project



  1. Wow, those are some really great photos of a great varve exposure. And you've got those "irony" concretions, too. I once (many years ago) found a similar (but not nearly as big) open exposure of glacial lake clay varves on Anthony Road, just off of Duer Road in southern Town of Fort Edward. The Ft. Miller topo map shows that you have real closeness of two different drainages patterns there: (1) the large, loopy Moses Kill drainage, and (2) just a few 100 ft away, the picture-perfect dendritic drainage of Slocum Creek. I think the differences are because the Moses Kill is cut into the messed-up, faulted Ordovician Snake Hill Shale, where the Slocum Creek tributaries are downcutting into flat, undisturbed Pleistocene lake clays. Regardless of that, this is a really great post, Don.

    1. Mike,
      Your intriguing comment sent me to the 'maps'. The Moses Kill seems to loop south to get around a 400' to 500' ridge that lies west of Argyle Village. I believe these hills are erosional remnants of Taconic flysch that are a little more resistant than the underlying shale. Could Slocum Creek eventually capture the Moses Kill? Gullies from the two systems almost connect on either side of East Road above Fort Miller. Pre- and post-glacial drainage patterns are a fascinating puzzle.
      Also wanted to compliment you on the remembrance of Anson Piper published locally earlier this year. A warm tribute to a fine man.

  2. Thanks very much, Don! (Isn't that a cool spot? You're exactly right!).