The Troumbly Gravel Pit, a gravel pit that is being actively mined contained within what is called the Holman Esker.  It is located east of the defunct mining town of Holman which is just east of Taconite.  An esker is a glacial formation that is formed within a "river" or channel in a glacier.  When ice from the glacier melts it easily travels to the base of the glacier and accumulates forming a river at the base of the glacier to carry away the meltwater.  With changing temperatures the amount of water that flows through this channel is also changing.  When the flow decreases with the temperature the water's ability to transport material also decreases so that the material is deposited in this channel.  When the glacier recedes the structure is left that looks like an inverted river.  An esker contains all kinds of glacial material from large boulders to silt and clay.  The material that is deposited reflects the direction of the advance of the glacier.  In the Holman Esker there is evidence of two glaciation areas, the Wadena Lobe and the St. Louis Sublobe.  Both of these glacier lobes were part of the Wisconsinan Glaciation Period with the Wadena Lobe occurring first followed later by the St. Louis Sublobe as shown in these diagrams.  The third diagram on the top row shows the Wadena Lobe and the middle diagram shows the St. Louis Sublobe, the small hooked section in northern Minnesota. (Ojakangas and Matsch, Minnesota Geology, 1982, p107)   MGS source for information on Minnesota Glaciers.
The first view getting off the bus.  Notice the large boulders as well as the very fine sand and about everything in between.  The boulders are well rounded showing the amount of abrasion before they were deposited here.  While the boulders have been piled here by the mine operators, the presence of both the large and small material is evidence of the lack of sorting that would occur in an outwash plain.  In a situation of an outwash plain larger material would settle out first extending to smaller material as the velocity of the water decreased. 
     This is a sample of pillow basalt.  Note the rounded patterns in the rock that indicate it was formed from lava that cooled quickly in the presence of water.  The light colored stripe at the left of the rock is an intrusion of granite.  This is a common occurrence in the basalt formations along the North Shore of Lake Superior.  Rocks of this type indicate that they were moved here from the north east, the direction of the St. Louis Sublobe. 
     This photo is of a large boulder of mostly gabbro with intrusions of granite.  Gabbro is large grained igneous rock formed deep inside the earth (intrusive) where cooling can occur slow enough to form the large crystals.

     The rock below right is a basaltic rock with granite intrusions, common in the Duluth Complex.

     The rocks above and right are quartzite, a metamorphic rock that is formed when sand (quartz) is pressed together with heat and pressure.  A similar rock of orthoquartzite is a sedimentary rock that is cemented with silica.  An example is Sioux Quartzite from southwestern Minnesota.  Additional information on Sioux Quartzite will be at the end of this presentation.
     This rock is a metamorphic rock, most probably Ely Greenstone.  Just because it has "Ely" in the name doesn't mean that it came from Ely, MN.  It is a classification of metamorphic rocks that have outcrops at the surface in Ely but also form the bedrock of much of northern and northwestern MN.  The bedrock is folded into synclinal structures with granulite gneisses and is crosscut by granite.  The Greenstone belts form linear trends from SW to NE across much of northwestern MN.  Komatiite is very distinctive of the greenstone belts.  It is a rock that is the volcanic equivalent of peridotite.  It becomes liquid about 1600oC, a much higher temperature than the basalt (1400oC) that is released from volcanoes today.  The greenstones also have a pillow formation.
     The Tower Soudan iron mine lies in these greenstone belts but all of the other mines that make up what is known as "The Range"  were formed differently and is the subject of our next two stops.  (Class notes from Dr. Kroeger)
   The photo at the left is of iron containing rock.  The iron bands (darker colored areas) are likely hematite while the reddish bands are jasper, a form of chert that contains iron oxides to give it the reddish color.  These are both forms of quartz.
     The rock at the right is an amphibole shist, also a metamorphic rock.  This particular sample has a large intrusion of feldspar (the arrows).

 

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