Glacial Geology of the ArboretumSUNDAY, JANUARY 26, 2014
The forecast was for cold and extremely windy weather, but when our program started at 1 o’clock, temperatures were in the 20’s and wind was under 5 mph. This was fortunate for the nearly twenty participants on our snow-laden hike. I began by thanking the Friends of the Arboretum for their generous support of our weekly free and open to the public programs. Additionally, I read a summary report from the Wisconsin Geologic Survey which I thought did a fine job of explaining and placing Dane County in a larger, historical and geological context.
The Ice Age of the past 1.5 million years had a huge hand in shaping and defining Dane County. Over this time, at least six different glaciation events took place and affected our area. Most recently, between 12,000 and 18,000 years ago, the Green Bay Lobe of the greater Laurentide Ice Sheet made its way across most, but not all, of the state. The so-called “Driftless Area” refers to this exact phenomenon. The Green Bay Lobe did not “drift” that far. It stagnated just west of Madison and left a noticeable terminal moraine. As a boy I always wondered about the large hill located just west of Shoveler’s Sink (another artifact of glacial work) along Mineral Point Road that needed to be ascended in order to travel from Pine Bluff and into Madison – this is exactly that, the terminal moraine. It is comprised of glacial till, or glacial sediment, that in this case marks the point of the glacier’s final resting place prior to receding back to the north.
Our first stop was to look out over Curtis Prairie and notice the slight bowl shape denoting that it had once been a small glacial lake. Next we headed up through the Native Garden and looked at one of the granite rocks that was transported to this area by the glacier. These foreign boulders are known as glacial erratics. Notably, those behind the visitor center were moved by earlier settlers looking to clear land for farming. When the CCC worked at the Arboretum in the 30’s, one of their many tasks and final products was to re-insert the nearby glacial erratics into the landscape.
Next we tromped through Longenecker Gardens taking note of a few of the individual ID tags located on each tree found there. At the far northwest end we began to climb the drumlin. A drumlin is one of many artifacts unique to the past-presence of a glacier. Their formation is less understood than others (for instance, eskers, which form a large “S-shaped” mound of deposited sediment from tunnel channels at the base of glaciers), but their shape is that of an elongated oval or half-buried egg. They actually point in the direction that the glacier traveled, so for this part of the program we were actually hiking a cross-section of the drumlin and not along the length of it. Drumlins tend to be found within the final fifty or so miles of a glacier’s advance. Northeastern Dane County has over a hundred. If you travel on Highway 151 northeast out of Madison, you will be traveling parallel to many of the drumlins. On the other hand, if you head straight east out of town on 90/94, you will be cutting over and through a number of drumlins.
We traveled on and crossed the road in order to enter Wingra Woods and hike down the far side of the drumlin and towards the Big Spring. At the Big Spring we took a few moments in awe of the bird life that abounded. A Great Blue Heron was perched over open water on a floating snag from a tree, a Belted Kingfisher called and then swooped in for a short perch only 25 feet from where we stood, Red-bellied Woodpeckers chased one another through the trees and American Robins continually flew across the open view above the water (which remains open year-round therefore providing a wonderful resource for animals and in turn wildlife watchers). I took the opportunity, as I often do at the Big Spring, to read a short passage from Aldo’s Sand County Almanac…
“There are two spiritual dangers in not owning a farm. One is the danger in supposing that breakfast comes from the grocery, and the other that heat comes from the furnace.”
On the way back, we took our time hiking the drumlin lengthwise before ducking back across the road, through Longenecker Horticultural Gardens and back to the Visitor Center.
Great Blue Heron
American Tree Sparrow
Wild Turkey (tracks in Longenecker and Visitor Parking Lot)
Respectfully submitted by David Laufenberg