The term "mound" in Wisconsin refers to any isolated hill. Some, like Necedah Mound or Hamilton Mound, are monadnocks of Precambrian rock. Others, like Blue Mounds or the Platte Mounds, are capped by Silurian outliers. But the term is most commonly applied to the castellated mounds, isolated hills of Cambrian sandstone rising steeply above the central lowlands, and occasionally capped by Ordovician dolomite. Usually, they simply consist of sandstone, and are often steep-sided pinnacles. They are far too delicate to have survived glaciation, and many owe their steepness to wave erosion by Glacial Lake Wisconsin. They are ephemeral features and will be gone in a few tens to hundreds of thousands of years.
The map below is redrawn from Lawrence Martin's 1932 The Physical Geology of Wisconsin, a classic of Wisconsin geology and loaded with keen and accurate observations. Precambrian rocks are in purple, Cambrian sandstone in white or gray, Prairie du Chien dolomite (early Ordovician) in light blue and Sinnipee Group (mid-late Ordovician) dolomite in green. Darker shades indicate glaciated areas. Note the relative absence of mounds in the glaciated areas except near Neillsville and in the center of the map, where large mounds were able to withstand the thin ice near the glacier margins. The few other mounds in the glaciated areas are inconspicuous, being nearly buried, and lack the delicate features of the mounds in the nonglaciated area.
It's interesting to note that nowadays we tend to think of Wisconsin as an eastern state, or at least midwestern, and don't call these features mesas or buttes, because those are Western landforms. We would not think twice about calling them mesas and buttes if they were in Wyoming. Martin unhesitatingly refers to them as mesas and buttes, and regards them as "the very frontier of the true West." (P. 317)
The map below identifies mounds by name. Martin's book was published at a time when topographic maps were not always available, and the locations and sizes of some mounds have been corrected. Note that some obvious names, like Castle Rock, may be applied to several hills. There are also several Wildcat Mounds, and other duplicate names may occur. Also, some features may have several local names. Some features not shown by Martin have been added.
Major Precambrian monadnocks are shown in purple, with purple letters:
Created 15 Sept 2000, Last Update 19 Sept 2000
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