ICE AGE Geology of Ice Age National Scientific Reserve of Wisconsin NPS Scientific Monograph No. 2

Sheboygan Marsh, almost 20 mile² in area, lies directly behind the front of the Green Bay Lobe where it abutted ice of the Lake Michigan Lobe to form the Kettle Interlobate Moraine in Sheboygan County (Figs. 1, 2, 4, 14, 15). Sheboygan River runs easterly through the center of the marsh which is partly occupied by Sheboygan Lake. Water level is controlled at 907 ft above sea level by a dam at the northeast corner of the marsh. Much of the marsh is a county park and wildlife refuge. One of the best views of the marsh (Fig. 15) is from the terminus of the town road which serves the house on the bluff on the north side of Sheboygan Lake, in the west center of sec. 14, T. 16 N., R. 20 E. A public overlook at this location is highly desirable.

Details of the history of the marsh, a former glacial lake, and of its deposits are lacking. Bedrock supported hills rise more than 100 ft above the marsh on much of its north, south, and west margins (Fig. 14). The Kettle Interlobate Moraine forms the east margin. Alden (1918:46, Pl. II) shows that a deep pre-glacial valley trended southeastward through Sheboygan Marsh and Elkhart Lake. On the west side of that lake a well 240 ft deep penetrated only unconsolidated materials. Seemingly, more than 100 ft of sediment fills Sheboygan Marsh, of which some at least is marl (Alden 1918:47). The Sheboygan River undoubtedly has contributed much material to the marsh, and an unknown but appreciable proportion probably came from the local vegetation added year after year since the late Woodfordian or Cary ice disappeared from the area. But why is the marsh almost filled while Elkhart Lake is still 113 ft deep? Was its buried ice that much thicker or has more detritus been brought to Sheboygan Marsh? Both surmises seem plausible, but the story remains to be worked out.

Fig. 14. Topographic map of Sheboygan Marsh, a former glacial lake, and its relationship to Kettle Interlobate moraine whose west border is shown by the heavy broken line. The position of a retreatal moraine of Woodfordian age is shown northwest of the marsh by ticked line. Directions of ice movement are indicated by long straight arrows and of water flow by short arrows; some local kames are indicated by the letter "k"; and two ice-walled lake deposits by the letters "iw". Part of the U.S. Geological Survey topographic map-Sheboygan.

Fig. 15. View southward of Sheboygan Marsh, from recommended overlook on the north side (Fig. 14).

What role either the Cary or Valders melt waters played in filling the marsh is not known. However, the marsh lay between the Valders ice that filled the area of Lake Winnebago to the northwest and of Lake Michigan to the east. Sheboygan River would have been dammed on the east, undoubtedly raising water levels in the area of the Marsh. Furthermore, the South Branch of the Manitowoc River would have had its outlet cut off by the Valders ice on the east, but at the same time it would have been receiving water from the Valders ice in the Lake Winnebago area. The divide between Sheboygan Marsh and the South Fork of the Manitowoc River to the west is only about 950 ft above sea level. It seems likely that both areas were once part of an extended glacial lake, but no research has been done on this problem.

The bedrock-supported hills that rise above the Marsh on all sides but the east show clearly by drumlinoid and fluted forms the southeasterly direction of flow of the ice of the Green Bay Lobe. During stagnation of the Cary ice, water flowed across and around a number of those hills. The most striking channels are north of Sheboygan Lake in sees. 14 and 15. Water from off the ice in what is now Elkhart Lake also flowed southwesterly. The area of Glenbeulah, northward and also southwestward to Greenbush in the Mullet River Valley, has the surficial appearance of having been "washed over." Sand of possible lacustrine origin is common in that area. A small morainal area of knob and swale topography lies west of Glenbeulah.

Around the Marsh in various places are small irregular ice-contact features. Most are kames; Alden (1918) mapped a small esker north of the Marsh but most of it has been removed. A few of these features are shown in Fig. 14. The small area enclosed by black line in Fig. 14 in the southeastern part of the Marsh appears from inspection of aerial photographs to be an ice-walled lake area (a former lake that received sediment from the enclosing glacial ice) produced during stagnation of the Cary ice. Another appears in the northeastern corner of the Marsh. No borings or excavations of these features or of the adjacent marsh sediments have been attempted.

Sheboygan Marsh thus is the final vestige of a large glacial lake into which many tens of feet of fill have been laid down by wash from the surrounding hillsides, from the Sheboygan River, and from the growth of vegetation within the lake. The details of its history have not been reconstructed. It seems typical of many large lakes that originated during the Woodfordian and Valderan glaciations. Others, like Horicon Marsh in Dodge County, are as far advanced or even farther in their maturation. Others, such as Lake Winnebago, are doomed ultimately to the same fate. Man usually in creases the rate of maturation; he has difficulty in retarding it.