Reading the LandscapeSUNDAY, NOVEMBER 15, 1998
Now that the leaves have fallen, and some of the tall prairie plants have been blown over, it’s a good time to see the contours of the landscape and try to imagine how they came to be the way theyare. There are fewer birds, and almost no flowers to distract us as we walk the Arboretum trails and think “Glacial Geology.”
12,000 to 15,000 years ago, a vast sheet of ice covered all of what is now the Arboretum. The “Wisconsinan” or “Woodfordian,” the most recent glacier, reached its greatest extent covering all but the southwestern quarter of Dane County. The portion of the ice sheet that reached this area is called the Green Bay Lobe, as it flowed here from the direction of the Green Bay lowlands.
As long as it existed, the glacial ice “flowed” toward the front of the glacier, even when climate caused melting of the front, or margin, at a rate faster than the ice was advancing. The ice acted like a combination of bulldozer and conveyor belt, continuously destroying the landscape in front of it and carrying “debris” (particles of all sizes, from huge boulders to minute gravels, sands, and clay) in the direction of its flow. As the climate warmed, and the ice margin melted back to the northeast, several features caused by the ice and the materials it released, were revealed.
At its greatest extent, the rate of melting and advance reached equilibrium for a prolonged period, resulting in the build-up of a ridge of glacial debris paralleling the ice margin. This “moraine” is known today as the Johnstown Moraine, and runs from northwestern Dane County, near Mazomanie, toward the southeast, near Brooklyn, passing closest to the Arboretum on the west side of Verona. Sometime later, as the final melting of the ice progressed, a second well-defined moraine formed. Known as the Milton Moraine, it lies between Verona and the Arboretum. When you drop from the vicinity of Nicolet Instruments on Highway 18/151 to County PD, you are coming down off the Milton Moraine.
For a time, the ice margin also lay across the southwestern portion of the Arboretum, from what is now western Nakoma, and Tony Frank’s, through the Arbor Hills area. During this period, another, smaller moraine built up the hill where Noe Woods, Evjue Pines, and the western portions of the Leopold Pines are located. You rise up onto the backside of this moraine as you pass the west end of Curtis Prairie and exit the Arboretum onto Seminole Highway.
Also during this stage, while the ice margin was relatively stable, some blocks of ice became detached from the main sheet. Covered by glacial debris, they gradually melted away, possibly well after the ice margin had melted back to the northeast. As they melted, and their moisture drained away, the debris slumped, forming “kettle holes.” One, lying south of the railroad corridor and west of Seminole Highway, is below the water table. We know it today as Dunn’s Marsh. Another well-defined kettle lies about the water table in the vicinity of trail marker #U2, just south of the Evjue Pines.
As the ice margin angled across this portion of the Arboretum, it blocked the outflow of its own melt water. For a period long enough to leave a telltale marking today’s sub-soil layers, a small “ice-marginal” lake formed in the low area, which is now Green Prairie. Over a number of years, fine glacial silts and clays settled to the bottom of “Glacial Lake Greene” forming a layer that can be seen by taking deep cores from below the soil level.
A second ice marginal lake formed over the area that is now Curtis Prairie at a time when the ice margin lay across the Nakoma Golf Course, right through the McKay Center, and on to the southeast toward American TV and the Beltline. Corings also confirm the existence of this “Glacial Lake Curtis.”
When the ice margin melted even further to the northeast, beyond the Arboretum, it exposed a changed hillside on which now sits the Wingra/Gallistel Parking Lot (markers # G8 and N4). What very likely was a sandstone cliff overlooking the valley now filled by Lake Wingra had been sculpted by the flowing ice, forming a “drumlin.”
Like all drumlins in this part of the county, the hill is steepest to the northeast, forming a teardrop that gradually tapers off to the southwest, the direction of ice flow. Mined by early settlers to the area, the sandstone bedrock can be seen in a small quarry in Wingra Wood to the northeast of the Indian Mounds at marker #K3. This sandstone is the “native” bedrock of this area, and differs greatly from the glacial “erratics” which dot the landscape throughout the Arboretum.
Blocked for a time by the debris of the Milton Moraine near Stoughton, the valley of the Yahara River in early post-glacial times was one vast lake, “Glacial Lake Yahara.” At this stage, a shoreline developed where wave and ice action eroded away sands and gravels leaving exposed a row of erratic boulders, much like that which can be seen along Picnic Point on Lake Mendota today. Once the Yahara River eroded through the moraine, the level of Glacial Lake Yahara quickly dropped several feet, but still encompassed the entire “Four Lakes” area. Both shorelines of Glacial Lake Yahara have been located in Wingra Woods.
Since then, the Yahara River has been very slowly eroding through bedrock. So the shores of the second stage of Glacial Lake Yahara are even better defined than the first. This second shoreline of boulders lies at almost exactly the same level as the trail that runs from Skunk Cabbage Bridge, near marker #K4 to the Big Spring, at N6. Next time you stub your toe as you walk the lower trail in Wingra Wood, just remember its all a gift of the glacier!
—Ken Wood, Arboretum Naturalist