Geology of the Ice Age National Scenic Trail

Superior Lobe Ice Age Trail Segments

B etween 25,000 and 30,000 cal. years ago, the Superior Lobe of the Laurentide Ice Sheet advanced into northwestern Wisconsin out of the Lake Superior basin. It advanced across a landscape over which, in turn, several other glaciers had slid and scraped in the past. The deposits from these older glaciers sit at the surface south of the late Wisconsin glacial deposits that the IAT segments cross (fig. 259; Baker et al. 1983). You will see till that the Superior Lobe deposited: it is reddish brown because of iron staining, similar to the till in other lobes that came out of the Lake Superior basin. There are substantial differences, however, between the rock types that make up the till from each successive glacier lobe. To explain why, we need to look at much earlier geologic history that shaped the rocks the glaciers later carved. Late in the Precambrian, about 1,100 million years ago, long northeast-southwesttrending fractures and faults developed as tectonic forces began to pull apart the North American continent (fig. 260). This tearing of the earth’s crust produced valleys similar to the rift valleys in East Africa today. The rift in this area of Wisconsin is called the Keweenawan, or Midcontinent, Rift, and it extends from the Lake Superior basin toward the southwest as far as Kansas (fig. 261). For the most part, it is covered south of Minnesota by younger sedimentary rocks. As the rifting occurred, volcanic eruptions of lava flowed into the new rift valleys. The valleys flooded time and again with hot lava, producing overlapping layers of basalt (SB 21) across the entire area. Streams of water flowing down the steep slopes on the sides of the rift valleys carried gravel and sand that accumulated on top of the basalt flows in many places. The resulting distribution of Keweenawan rocks is shown in figure 236. You’ll 336 Superior Lobe Ice Age Trail Segments Figure 259. Shaded relief of the Superior Lobe IAT segments (red): (115) Bear Lake, (116) Grassy Lake, (117) Timberland Hills Area, (118) Sand Creek, (119) McKenzie Creek, (120) Pine Lake, (121) Straight River, (122) Straight Lake, (123) Trade River, (124) Gandy Dancer State Trail, (125) St. Croix Falls. The Interstate State Park Unit of the Ice Age National Scientific Reserve (126) is located at the western terminus of the Ice Age Trail. Yellow lines and numbers indicate major highways. Cities and villages shown (yellow): (B) Baronette, (Ba) Barron, (C) Centuria, (Cu) Cumberland, (F) Frederic, (M) Milltown, (R) Rice Lake, (SCF) St. Croix Falls, (TL) Turtle Lake. Dashed blue line SC is outer edge of the St. Croix advance of the Superior Lobe. Dashed blue line E is approximate outermost extent of the Emerald Phase of the Superior Lobe. Dashed blue line C is approximate outermost extent of the Centuria Phase of the Superior Lobe. Dashed blue line (PC) is the approximate outermost extent of the Pine City Phase of the Grantsburg Sublobe. Blue arrows show ice-flow direction. (Base map constructed from USGS National Elevation Dataset and modified by WGNHS.) see the exposed basalt flows on the IAT near its western terminus at Interstate State Park, especially along the walls of the gorge. In places, air bubbles were trapped in the lava as it cooled and hardened. Groundwater rich in dissolved calcium, silica, iron, and other elements later flowed through these openings , forming crystals that partly or completely filled the bubbles. Common crystal fillings you can see today include the minerals calcite, quartz, and feldspar. Many hobbyists in northwestern Wisconsin and Minnesota collect Lake Superior agates. These brightly colored stones formed when concentric bands of very small crystals of the mineral quartz (SiO2), variously stained with iron, accumulated in voids in the basalt. Weathering and erosion for tens of millions of years released these weather-resistant pieces of agate, and now you’re likely to find these relatively common stones in the till or sand and gravel of the Superior Lobe. Glacial deposits overlie Cambrian sandstone beneath trail segments east of the Straight River Segment, but basalt boulders that the glacier carried from the northeast are present all along the trail. From there to the western terminus of the IAT, the trail overlies Keweenawan basalt. The earliest late Wisconsin advance of the Superior Lobe is called the Emerald advance (fig. 259). The glacier deposited a thin, discontinuous till layer without building an...