Geology of the Ice Age National Scenic Trail

Western Green Bay Lobe Ice Age Trail Segments

T he nature of glacial deposits changes dramatically as you walk northward along the western edge of the former Green Bay Lobe. You’ll be able to spot many obvious tunnel channels (SB 17) here because there are fewer bedrock hills to confuse with glacial landforms than there are in the southern Green Bay Lobe. You’ll see the exception to this in a few places where residual bedrock hills stick up through the glacial sediment. As you hike these segments you’ll also see end moraines (SB 6) and outwash heads (SB 10), which are easier to recognize here than they are in much of southern Wisconsin. In the late Wisconsin glacial advance, the Green Bay Lobe scraped and scoured the landscape , leaving drumlins (SB 14) as reminders of the glacier’s path and delivering sediment to the ice edge. In this part of Wisconsin, you can see signs of the outermost extent of this lobe: the Johnstown Moraine (SB 6) stands above the surrounding landscape and contains the accumulated debris that the massive ice sheet carried to its edge (fig. 183). This moraine continues northward almost to the village of Coloma (C on fig. 183), where it appears to split into two moraines: the Hancock and the Almond moraines. Most interpretations, and the one shown in figure 184, indicate that the Hancock Moraine is older than the Johnstown Moraine, whereas the Almond and Johnstown moraines are about the same age. We can’t prove this, however, because the till (SB 5) in all three moraines is very similar and there is no way to distinguish one from the other except spatially. Presumably all are older than the Milton Moraine in southern Wisconsin, but there is little continuity of the Milton Moraine north of Portage, so their relative ages cannot be demonstrated either. The Greenwood, Bohn Lake, and Deerfield segments are all in the Almond Moraine and as a result have lots of ups and downs (fig. 183). 240 Western Green Bay Lobe Ice Age Trail Segments Figure 183. Shaded relief of the western Green Bay Lobe IAT segments (red): (71) Chaffee Creek, (72) Wedde Creek, (73) Mecan River, (74) Greenwood, (75) Bohn Lake, (76) Deerfield, (77) Belmont– Emmons–Hartman Creek, (78) Waupaca River (south and north), (79) Skunk and Foster Lakes, (80) New Hope–Iola Ski Hill. Yellow lines and numbers indicate major highways. Cities and villages shown (yellow): (C) Coloma, (I) Iola, (P) Plainfield, (SP) Stevens Point, (W) Wautoma, (Wa) Waupaca. Prominent landforms are labeled. Blue arrows show ice-flow direction. Discontinuous Elderon ice-margin positions occur from the northern to southern edges of the map in the zone indicated. (Base map constructed from USGS National Elevation Dataset and modified by WGNHS.) Behind the distinct ridges of the Johnstown, Almond, and Hancock moraines are a series of irregular, discontinuous ridges. These form steps in the landscape that indicate pauses in the retreat of the ice margin or even small readvances. These irregular ridges are referred to as the Elderon Moraine system, although they are typically outwash heads (SB 8) rather than actual moraines. The ridges presumably are about the same age as the Lake Mills Moraine and perhaps the Milton Moraine, both of which occur farther south. Figure 185 shows a profile of the land surface from just west of Coloma (C on fig. 183) to the city of Wautoma (W on fig. 183). Note that when the ice edge sat at the Hancock and Almond moraines, the land sloped away from the glacier. Water flowed down this slope 242 Western Green Bay Lobe Ice Age Trail Segments Figure 184. Moraines (black), tunnel channels (green), and drumlins (blue) along the west edge of the area covered by the Green Bay Lobe in central Wisconsin. Arnott, Elderon, and younger moraines not shown. Area is somewhat larger than that shown in figure 183. (Modified from Clayton et al. 1999; drafted by Mary Diman.) toward the west across an outwash plain and into glacial Lake Wisconsin. Once the glacier’s edge began to retreat from the Almond Moraine, the land sloped back toward the glacier, to the east. Meltwater could not flow directly away from the ice anymore but instead ponded in valleys or flowed on outwash plains and in channels toward the south, roughly parallel with the ice margin. In many places braided streams (SB 8) deposited substantial thicknesses of...