Lakeshore Bluffs -windows into our ice age past

Photo of a drumlin bluff near Port Bay taken October 30

In my book published by History Press last summer, “The Natural History of Lake Ontario” I wrote about the ice age formation of drumlins in central New York and their presence along the lake’s south shore. The following article is based on an excerpt from that book.

The stretch of coast between Sodus Bay and Oswego contains a number of prominent clay bluffs. These form where a drumlin hill intersects the shoreline and begins to erode. Where they do so, they allow us to peer beneath the surface into the subterranean landscape to provide a unique view of ice age action upon our landscape.

Chimney and Sitts Bluffs, both on State Park lands, are the best known places to see what the interior of a drumlin looks like. And there are several other bluffs on public lands in Wayne and Cayuga county where you can peer into earth’s past. At first glance, these cliffs of clay spiked with stones ranging from pebble to massive boulder size, seem fairly nondescript. But if you study them a bit, they provide some intriguing features that hint at events from ten thousand years ago.

Photo of Sitts Bluff-note the faint bands suggesting layers of deposition. Possibly this happened as the ice advanced and retreated during the last glaciation.

The lake shore bluffs are made up of a mix of clay, sand, and rock called glacial till. Clay forms over unimaginably vast stretches of geologic “deep time” as igneous rock weathers away. The sedimentary clays of the bluffs are made up of flat plate like grains. These particles slip and slide against each other readily resulting in a “plastic” material that is easily shaped. This makes a clay well suited for pot making. It is also a material that erodes, slides and slumps readily when wet.

Typically clay and other material flows and slumps down from the top of the bluff bank and is then washed away by wave action-this photo was taken in March, a prime time for slumps.

The bluffs actively erode in their natural state and recede at varying rates depending on the season, their location and the lake’s water levels. Spring, when frozen ground thaws and heavy rains may occur, is usually the peak time for erosion. This is when massive slumps sometimes occur as tons of clay sheer off and fall onto the beach or into the water.

Large chunks have ‘flaked’ off this bluff face to land on fast melting anchor ice -photo taken March 2021

The freshly exposed clay left from a slump often shows variations in color and texture. Veins of blue clay, gravel or sand are more easily seen after a big chunk ‘flakes’ off. Different conditions prevailed during the clay’s long ago formation to cause the colors- blue clay for example forms under conditions of less oxygen while the pinkish or reddish clay is colored by iron oxide ( rust). White or yellowish clay lacks iron oxide. Some clays change color as they weather, so the freshly exposed bluff face may well look different from the rest of the surface.

Note distinct layers and colors seen here in freshly exposed clay- they will quickly disappear with weathering.

On some bluffs you may see a dark band of color where the clay is wetter than surrounding bluff face. Often it is quite pronounced in the spring, and may fade later in the summer. I assume this represents an area where ground water contained within a thin aquifer is visible on the surface of the bluff face. We first noticed such a ‘wet’ area on a bluff face some years ago and realized that it corresponded closely with the depth of a nearby hand dug well. It shows how materials within the drumlin vary. Ground water accumulates in sediments like gravel and sand that have greater porosity and permeability than clay has.

The dark streak across the face of this bluff is wet clay, presumably from ground water seepage

Last summer we anchored near a limestone bluff face on Prince Edward County. When I paddled the dinghy close to it I was surprised by water dripping off a stone ledge overhead and pattering into the water around me. The dark color in the photo below is wet stone and there also seemed to be more vegetation growing on the stone in the seep area. Clearly ground water was flowing out of this part of the limestone formation. It’s harder to see that happen on clay bluffs, but I have seen a damp gully on the face of Chimney Bluff when it has been weeks since a heavy rain that could have contributed runoff.

The ground water seep coming out of Little Bluff ( South Bay off Prince Edward Bay) is visible as a darker area. In June 2022 the water was actively dripping off a protruding rock.

I learned while researching my “A Natural History” book that ground water interaction with Lake Ontario is not well understood. It does affect the lake, however, and helps stabilize the marshes and wetlands along our lake shore. The total volume of groundwater within the Great Lakes basin is probably about equal to that contained within Lake Michigan according to some hydrogeology studies.

I was told by a Canadian geologist that these odd little holes and hollows along the bluff’s lower portion that appeared during the two past high water years were probably formed by groundwater flows. And I have several times encountered springs at the base of various bluffs.

Although erosion of the lake’s higher shoreline usually occurs from the top down, when lake levels are high, wave action can undercut a bluff’s base. Two high lake level years and a reduced supply of sediment available for beach building have changed this bluff’s face shape seen below considerably. Normally the beach absorbs a lot of the wave energy that would otherwise erode the shore. But with little beach, this bluff was severely undercut. After extensive undercutting it has assumed an atypical angle compared to that of other adjacent bluffs. When next spring’s thaws arrive, massive slumps of the vertical clay will again occur.

note narrow beach and vertical slope on this bluff seen Oct 30, 2022. Lake levels were very low and other bluffs nearby had wider beaches.

For more information on drumlins and shoreline dynamics check out “The Natural History of Lake Ontario” available on line at Amazon or on my shop at Etsy link or at Rivers End Books of Oswego. And watch for upcoming special deals on my other titles for gift giving! buy my new book here

Below is a drone photo taken a couple of days before my Oct 30 canoe excursion. Now the trees are nearly bare as they await winter gales.