Pillsbury - 1
Pillsbury Hall
With its walls of massive sandstone blocks, Pillsbury Hall is a fitting home for the Department of Geology & Geophysics. The University’s original science building and second oldest structure, Pillsbury Hall also played a role in keeping the University of Minnesota a two-campus institution. Before its construction, there had been a movement towards spinning the St. Paul campus off as a separate agricultural college. John Pillsbury, a former Minnesota governor, opposed this idea. When fire damaged the science building during construction, Pillsbury donated funds for its repair, but on the condition that the two campuses could never be separated.
ROCK GARDEN
Located on Pillsbury’s northwest corner, the Rock Garden holds an eclectic mixture of mineral and rock samples. Early department faculty brought the larger pieces here to use as teaching samples. Over the years though, the rock garden was forgotten and neglected. University grounds workers recently ‘rediscovered’ the area, trimming foliage and adding paths so students could again visit the rocks.
COLUMN
A green rock column dominates the Rock Garden’s center. Known as ‘Ely Greenstone’, this column was originally cut during excavation of a new ventilation shaft for an Iron Range mine (near Lake Vermillion).
The Ely Greenstone originally formed as part of an ancient seafloor. Over 2.7 billion years old, it formed at a time when small micro-continents were still coming together to assemble North America. Born from volcanism at an ancient spreading zone, slivers of ocean crust were later caught between converging micro-continents and slightly metamorphosed.
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You can tell that this rock formed as an ancient seafloor, because of the rounded, elliptical shapes on the column surface. These shapes are called ‘pillows’ and they formed as basaltic lava erupted from a fissure on the seafloor. As molten lava came in contact with cold seawater, its outer surface quickly cooled to form a skin (the pillow’s outline). A succession of pillows formed as more lava broke through cooled pillow skins to again meet cold seawater. Pillow lavas only form from underwater eruptions and rock similar to this column forms the upper layer of the oceans’ present seafloor.
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Closer yet, the pillows become more obvious. If this rock, greenstone, is metamorphosed to a greater extent it becomes serpentine - a rock prized as a decorative building stone.
DOLOMITE
Our faithful html programmer is leaning against a stone that is almost completely composed of the mineral dolomite. In an admittedly non-creative vein, rocks primarily formed of dolomite are called dolostone.
This sample of dolostone came from the Taylor Falls area on the border between Minnesota and Wisconsin. It is from a rock unit called the "Oneota Dolomite".
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Although they are difficult to pick out, bedding planes can be seen on the rock’s exterior. Bedding planes are surfaces that reflect interruptions in the rock’s original deposition. They represent periods of missing time that might be measured in seconds, years, or even millennia.
Dolostone usually forms as an alteration of limestone. As magnesium-rich water moves through a limestone unit, some of the limestone’s calcite is converted to dolomite.
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As mentioned before, this rock sample came from the Taylor Falls area, near Interstate Park on the Minnesota-Wisconsin border. The depressions on the dolostone’s upper surfaces are potholes, scoured by turbulent water eddies. As water flowed across the rock surface, mud and silt carried by the water swirled in eddies, eroding these small basins.
If you visit Taylor Falls, be sure to stop at Interstate Park to see massive potholes eroded in igneous basalt meters across and up to twenty meters deep. These potholes are the most visible vestiges of a long-vanished waterfall that carved that part of the St. Croix River valley.
OUTSIDE
Castle-like Pillsbury Hall is built of two varieties of sandstone blocks. The lighter colored buff sandstone is called Hinckley Sandstone while the darker, reddish-brown sandstone is known as Fond du Lac Sandstone. Each of the rocks displays distinctive characteristics that reflect their origins in different depositional environments.
2 SANDSTONES
Both the Hinckley and Fond du Lac Sandstones are native to Minnesota. The blocks that make up Pillsbury’s walls came from quarries north of the Twin Cities area (near Sandstone and Hinckley) but quarries and outcrops of the units extend up to the southern shore of Lake Superior.
Blocks of Fond du Lac Sandstone dominate the lower and upper floors of Pillsbury Hall, while the Hinckley was used for the middle floor. Alternating blocks of the two are used for the decorative tier just below the roof.
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Even from a distance, the distinction between the Hinckley and Fond du Lac is obvious. Their colors reflect the sandstones’ different compositions and origins.
Feldspar grains give the Fond du Lac Sandstone a reddish hue. As feldspars break down quickly at the Earth’s surface, feldspar-rich sands usually form from a single episode of weathering and erosion, rather than multiple cycles. Over a billion years ago, North America had been rifting apart, but the spreading halted to leave behind a rift valley. Rivers filled this valley with deposits that became the Fond du Lac Sandstone. Deposited quickly, with little opportunity to be reworked, these sands remained feldspar rich. ‘Arkose’ is the geologic name for feldspar-rich sandstone.
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In contrast to the surrounding red Fond du Lac Sandstone, quartz grains make up more than 95% of the buff-colored Hinckley Sandstone blocks. Quartz is a resistant mineral that is concentrated in sedimentary deposits as chemical weathering breaks down other grains. Sandstones rich in quartz have undergone extensive weathering, either in tropical settings or during multiple cycles of weathering and erosion.
The Hinckley formed in the same area as the Fond du Lac, but at a later time, when the rift valley was almost completely filled. Quartz-rich sands formed in lakes, where waves, currents and time broke down less-resistant grains to create an almost pure quartz sand.
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Don't be fooled by the white specks in the Fond du Lac Sandstone. These are not fragments of Hinckley Sandstone, but are only areas where iron impurities in the sandstone were chemically reduced, rather than oxidized (rusted). They are simply a visible reminder that later processes can alter the rock’s original appearance.
In a similar fashion, do not be thrown by darker areas on blocks of the Hinckley Sandstone. These are areas where weathering and exposure discolored the rock. For decades, smoke from a heating plant that once stood behind Pillsbury Hall darkened the building. Pillsbury’s multiple colors and decorative patterns were only rediscovered after the building was sandblasted and cleaned in 1985.
SEDIMENTARY FEATURES
Sedimentary rocks that form from particles of pre-existing rock, like these sandstones, often contain features that reflect their depositional origin. Here gently sloping lines cut across the rock surface. These lines are the edges of planes that run through the rock, which formed as sand waves moved along the bottom of an ancient stream floor. Geologists call these features 'bedding' or 'bedding planes'.
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Bedding represents breaks in deposition or periods of erosion. Sedimentary rocks tend to part or break along these layers.
There are two general types of bedding: plane bedding and cross bedding. In plane bedding, horizontal layers of sediment parallel one another. These can form from sediment settling out of the water column, or when water is flowing very quickly.
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Cross-bedding formed as water or wind currents moved sand waves. Sand cascaded down the front of the shifting sand wave to form sets of curved surfaces that lie at an angle to the rock’s main, usually horizontal, bedding planes.
In areas of faulting and deformation, it is not always easy to tell which side of a rock layer was originally ‘up’. Cross-bedding is one way to determine the original ‘up’ direction, as the sets of curving lines are truncated at their top and merge together along their bottom. In the photo, the block above the pen was put in upside-down!
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Sometimes, the angle at which an outcrop or block is cut, can make cross-bedding look much more complicated than it really is. This is the case in a few of the blocks above the hand in this photo.
Although the bedding appears distorted, you should be able to use the cross-bedding’s truncation to determine that the block immediately above the hand is also an ‘upside-down’ block.
GRAINSIZE
Apart from their color, most of Pillsbury Hall’s sandstone blocks are composed of sand grains that are fairly similar in size. A few blocks though, contain scattered larger grains like the one in the photograph‘s center.
The white spots are not grains, however, but small areas of chemical alteration.
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Some of the Fond du Lac blocks contain layers rich in coarser grains. These represent periods when greater energy allowed water currents to carry, and eventually deposit, larger grains.
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Another block of Fond du Lac Sandstone that hosts layers of coarse grains. This block can be found along Pillsbury’s northwest stairway. This block is oriented so that the photo is looking down on a ‘map view’ of the bedding planes, rather than the side views common to most of the building’s sandstone blocks.