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Picnic Point Rock Wall

Key to the Glacial Erratics in the Rock Wall at Picnic Point Entrance

 by Phil Brown (UW Geosciences), David Liebl (UW Engineering), David Mickelson (UW Geosciences)  - © 2017
Built of glacial erratics the fieldstone wall at the entrance to Picnic Point provides visitors, students and others an opportunity to learn about Wisconsin’s geology. This key identifies the types of rocks and minerals visible in the roadside face of the wall, and discusses how these fieldstones came to be found in Dane County. 
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Extending along University Bay Drive on either side of the Picnic Point service drive entrance, a fieldstone wall faces the 1918 Marsh and the west campus medical complex. [ 43° 5.090'N, 89° 25.732'W]. Parking is available in Lot 130 [Lot 130 map]
      In 1925 Edward J. Young purchased the 110 acre R.J. Stevens farm which included Picnic Point. The fieldstone wall and entrance gate were built along University Bay Drive sometime in the late 1920’s. According to Alice (Mrs.) Young, the rocks used to build the wall were hand picked from farm fields in the vicinity of Cross Plains. It is our good fortune that the masons who built the wall split the stones to create a flat roadside face, thus revealing the rock’s interior characteristics.
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David Mickelson, Philip Brown, David Liebl analyzing the rocks. Photo G. Kutzbach
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Source of erratics, Green Bay Lobe. Map by David Mickelson
See also these related links:
http://wgnhs.uwex.edu/pubs/es043plate01/
http://wgnhs.uwex.edu/pubs/download_es043/
https://lakeshorepreserve.wisc.edu/geology.htm
http://www.friendslakeshorepreserve.com/geology1.html
      Fieldstones are still a common feature in many parts of rural Wisconsin, but less so near cities and towns where they have been gathered and used as building material. That many fieldstones did not come from the bedrock in the location where they were found (i.e. were “erratic”) led geologist Louis Agassiz in 1840 to deduce that they had been carried from far afield by glaciers. Once rough pieces of igneous, metamorphic or sedimentary bedrock, the erratics were shaped by weathering and the erosive forces of ice and water into more rounded rocks. Since the late 1800’s, geologists from the University of Wisconsin and others have worked to understand how glaciers have affected Wisconsin’s landscape, providing insights about where these stones may have come from.

       Between 29,00 and 18,000 years ago a glacier (the Green Bay Lobe of the Laurentide Ice Sheet) scoured the landscape of Dane County. Originating in what is now Ontario Canada, the Green Bay Lobe picked up pieces of bedrock from along its path and carried an assortment of them into Dane County where they were deposited as the glacier melted and withdrew. The white lines in the figure at left show the likely sources of the erratics as they were carried southward by the glacier. As the glacier edge began to retreat from Dane County about 18,000 years ago, the erratics that had accumulated on and at at the front of the ice sheet were deposited near its leading edge in a terminal moraine. In Dane County this outermost moraine, the Johnstown Moraine, runs from Brooklyn in the south through Verona and Cross Plains and to near Prairie du Sac in the north. As the ice sheet withdrew, erratics of all sizes lay across the landscape and many were cleared from farm fields and used for building.
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       The panorama of the east arm of the wall indicates the location of the section photos that follow. Use distinctive specimens or patterns of stones to help identify where the sections end and begin. Examples of the types of rock and rocks with interesting features are circled and labeled. Following each of the five wall sections are detailed pictures of some of the more interesting rocks. A glossary of geological and mineralogical terminology briefly explains the characteristics of the rocks, with links to the Wisconsin Geological and Natural History Survey – Minerals of Wisconsin database.
 Section 1
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Section 2
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Section 3
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Section 4
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Section 5
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Term                            Description [Dates in Ma (= million years ago) are for these samples] 
Igneous                        Rocks and minerals solidified from molten magma.
     Basalt                       Dark, finely crystalline, silica poor, formed from magma cooling rapidly at or
                                      near the Earth’s surface in massive flows, dikes or sills. Also called “trap rock”. 1100-3000Ma
     Gabbro                     Dark, coarsely crystalline, silica poor, solidified slowly within the Earth. Similar in composition
                                      to basalt  – original mineralogy feldspar + pyroxene. 2500-3000Ma
     Rhyolite                    Light colored, finely crystalline (or porphyritic), silica-rich, formed from magma cooling
                                      rapidly at or near the Earth’s surface. 1100 Ma 
     Granite                     Light colored, coarsely crystalline, silica-rich with a wide variety of colors and textures.
                                      A common igneous rock in the continental crust.


Sedimentary                Rock formed from the consolidation of layered sediments, fragments of other rocks,
                                      plant and animal residues or chemical precipitation.

      Chert                       Hard, extremely fine-grained quartz (SiO2). Formed as deposits in limestone and dolomite,
                                      and may appear as nodules or in beds. Flint is a variety of chert.
 480Ma
      Dolomite                  A common carbonate rock primarily composed of the mineral of the same name.
                                      Generally formed by partial replacement of calcite in limestone, forming dolomite.
                                      Local bedrock visible on top of Eagle Heights Woods. 480Ma
      Sandstone               Visible grains of quartz cemented together by silica, calcite or iron oxides. Hard to soft and friable,
                                      it may also contain feldspar or mica, and is found in a wide range of colors. Originally deposited by
                                      either water or wind. Local bedrock visible at the water line at Raymer’s Cove. 490-500Ma
      Turbidite                  Sediment originally deposited by underwater slumps or landslides. The resulting layers vary
                                      (grain size, composition) with distance from source. 1800-2300Ma

Metamorphic                Rocks that have been modified from their original type (igneous or sedimentary) by heat,  
                                       pressure, and fluids deep in the Earth’s crust.

      Amphibolite              Formed from igneous rock (e.g. basalt), typically dark and coarse textured.
      Gneiss                      Formed from both igneous and sedimentary parent rock. Characterized by coarse-grained
                                       layers composed of different minerals that may be folded.   2500-3000 Ma.
      Quartzite                  Hard, quartz-rich rock (originally sandstone) fused with silica by heat and pressure.
                                       Commonly white-gray to red-purple.
      Schist                       Formed from a variety of source rocks and minerals. Coarse-grained with  visible crystals
                                       and easily split especially along mica-rich layers.
      Slate                         Formed from clays and shales. Dark, fine grained and layered, easily split into sheets.
​
Minerals                         Natural inorganic elements and compounds as crystals formed by geologic/geochemical
                                        processes. Most rocks are composed of minerals.

      Amphibole                 A group of closely related rock-forming silicate minerals such as Hornblende.
                                        See also: http://wgnhs.uwex.edu/minerals/hornblende/
      Calcite                       A form of calcium carbonate (CaCO3), usually milky white to clear. A common component of
                                        sedimentary rock, it is much softer than quartz. See also: http://wgnhs.uwex.edu/minerals/calcite/
       Dolomite                   Calcium-magnesium carbonate (CaMg(CO3)2), usually translucent white.
                                        See also: http://wgnhs.uwex.edu/minerals/dolomite/
       Epidote                     A light to dark green silicate mineral that can form hydrothermally in metamorphic or igneous rocks.
                                        See also: http://wgnhs.uwex.edu/minerals/epidote/
       Feldspar                   The most abundant group of minerals in the Earth’s crust. Crystals may be white to pinkish amber
                                        and fracture to rectangular form. See also: http://wgnhs.uwex.edu/minerals/potassiumfeldspar/
       Garnet                      A group of silicate minerals. Hard, brittle, lustrous and transparent, commonly red.
       Mica                         A group of silicate minerals that split cleanly yielding sheets or plates. Lustrous and
                                        transparent to black.
       Pyroxene                  A group of closely related rock-forming silicate minerals such as Augite.
                                        See also: http://wgnhs.uwex.edu/minerals/augite/
       Quartz                      One crystalline form of SiO2, clear to milky, white to black and very hard.
                                        See also: http://wgnhs.uwex.edu/minerals/quartz/
       Silica                        SiO2 is found in several crystalline forms. Term is also used to refer to relative amount of silicon
                                        found in magmas and igneous rocks (e.g. silica-rich).
OTHER TERMS
       Cross-cutting veins         Veins that intersect one another can be used to determine relative ages of
                                               vein filling events. May be accentuated by differential weathering.
       Concretion                      Formations superimposed on the host rock, cemented by silica, carbonate or iron oxides that  
                                               resist weathering, thus revealing odd shapes and textures as the surrounding softer material
                                               is weathered away.
       Differential weathering    Removal by physical or chemical processes of components of a rock by preference to
                                               their hardness or chemical reactivity, resulting in an uneven surface.
       Glacial striations             Scratches or other marks created on either bedrock or stones by the forces of 
                                               moving ice sheets. In bedrock outcrops these striations indicate the direction of ice movement.
       Graded beds                   A layer of sedimentary rock (e.g. turbidite) where the particle size in a layer ranges
                                               from coarse at the base to fine at the top of each layer (bed).
       Inclusion                          A fragment of an older rock incorporated in an igneous rock.
       Porphyritic                       The texture of igneous rocks that have larger crystals surrounded by tiny crystals
                                               (or even glass).
       Silicified Carbonate         Rock that has been altered by increasing the amount of silica (and consequently
                                               increasing the hardness).
       Vein                                 A fracture in rock that subsequently fills with one or more other mineral(s).
References:
Glossary of Geology - 5th Ed., American Geosciences Institute, 2011
Smithsonian Handbook of Rocks and Minerals, Smithsonian Institute, 2002
Cambridge Guide to Minerals Rocks and Fossils, Cambridge University Press, 2001



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Related websites:
UW Nelson Institute
UW Arboretum
Clean Lakes Alliance
Groundswell Conservancy
Pleasant Valley Conservancy
Pheasant Branch Conservancy
Friends of Amphibians
​Friends of Cherokee Marsh
Friends of Olin Turville

Wild Warner Park
  • Home
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