A majestic white rock that defies intuition is exposed in various locations throughout the Midwestern United States. It forms towering cliffs and bluffs that can be several hundred feet thick and completely vertical. It is so stable that it can support a dense network of caves, mines, and voids, yet so soft that it can be dug by hand! What is this strange rock unit, and why is it important?
The ease with which this unit can be removed is incredible. A layer containing trace amounts of iron are visible as the red color in the outcrop.
The Saint Peter is a unit of sandstone that occurs in the central US continent. It was originally deposited during the Ordivician period, approximately 460 million years ago. At this time, there was a shallow inland sea covering the center of the continent, resulting in a large sequence of clean, homogeneous sands being produced by wave and wind action.
Distribution & Exposure
Source | Distribution of the Saint Peter Sandstone. Deposits at or near the surface are shown in black, while subterranean layers are indicated by the dashed areas.
The Saint Peter formation has been identified in recovered cores throughout the Central US, and notably outcrops in Missouri, Illinois, Iowa, Wisconsin, Michigan, Arkansas, and Minnesota. Exposures in the more northern states generally occur along river valleys due to the prevalence of glacial drift and outwash in these areas. These river valleys were scoured out by torrents of water at the end of the last glaciation when the continental glaciers melted and glacial lakes (such as Lake Aggassiz) were drained.
View from an overlook showing the deeply incised Minnesota River Valley and steep valley wall covered in spring-fed ice.
The Saint Peter Sandstone is named after the Saint Peter River, a waterway that is now known as the Minnesota River. In southern Minnesota along this river and larger cousin the Mississippi, there are several outcroppings of the sandstone in the deeply incised glacial drainages. After glacial waters receded, the depth of this valley incision lowered the water table relative to the surrounding highlands, causing springs and natural caves to develop in the bluffs.
Railroad track hugging a Saint Peter bluff. Railroad companies tend to route through river valleys because of the relatively gentle grade.
This deposit is also sometimes referred to as Ottawa Sand due to it's exposure at Ottawa, Illinois. Ottawa Sand is extremely useful as a standard for scientific experiments because once sieved the grains are very uniform and can be used in calibration.
The Saint Peter sandstone has a quartz (SiO2) purity of more than 95%, often reaching a purity greater than 99%. Because of this lack of impurities, it is recognized as an excellent source of silica for creating glass.
Saint Peter outcropping in a small urban park. Notice the generations of initials etched in the outcrop and the surface discoloration due to moss, algae, and other microorganisms. A fresh surface of the pure, white sand is visible in the left side of the photo.
Roundness and Sorting
The grains of the Saint Peter are uniformly small in size and generally well rounded. The grains have a frosted appearance on their surface, indicating that they were tumbled, weathered, and reworked (most likely by wind) for an extraordinarily long time. The nature of the grains of this well sorted unit increases the grain-to-grain contact of the sands. This mass of hard silica grains can support an extreme amount of weight without failing.
Macro view of Saint Peter sands with a toothpick for scale. Notice how rounded and frosted the grains are, similar to the cover photo.
Grain CementationWhile the grains may be extremely uniform in size and distribution, they are often weakly cemented with a calcite or silica based cement. In many northern locations, the stone yields in a satisfying crumble upon being touched. This is especially true in deposits near the surface where water infiltrates the rock and any existing cement can be leached. The addition of a tool such as a pickaxe or pressurized water makes this rock extremely easy for an individual to excavate.
In Minnesota there is a rich history surrounding the Saint Peter Sandstone and its many uses. The following are just a few examples of how this immaculate unit has been utilized through time.
Source | Image from 1913 showing a staged photograph of Native Americans at the entrance of Carver's Cave after it was reopened.
Native AmericansThe white sandstone bluffs along the Mississippi and Minnesota Rivers have been inhabited by Native Americans for thousands of years. Groundwater action and the lack of cement in the sandstone has led to the formation of several caves along the river valleys. The most famous of these caves is arguably Carver's Cave, first described by Jonathan Carver in 1767. The cave was referred to as Wakan Tipi by the Natives and was the site of an early treaty negotiated by Carver. The Natives had ceremonies and meetings in the cave but railroad expansion has destroyed the original entrance.
Source | Image showing Fountain Cave, the site of the first Pioneer settlement in the Twin Cities area.
European SettlersSome of earliest non-native settlers of Minnesota were French fur trappers and soldiers at Fort Snelling, a historical US military stronghold perched atop a Saint Peter Sandstone bluff at the confluence of the Minnesota and Mississippi rivers. A nearby cave along the Mississippi River known as Fountain Cave was the site in 1838 of the first staked claim in what is now St. Paul, MN. A lively character known as Pierre "Pigs Eye" Parrant settled this area to create a whiskey bootlegging still. The cave was subsequently used as a tourist attraction until eventually being lost in a road construction project in 1964.
German Beer Lagering
Though Saint Paul's first settler may have been distilling whiskey, many subsequent settlers favored the Saint Peter Sandstone caves for producing a slightly softer beverage - beer. Hundreds of breweries were founded in Saint Paul, Minneapolis, and other Minnesota towns during the mid- to late-nineteenth century, often by German immigrants. The year-round moderate temperature of caves in this area, stable at approximately 50 degrees Fahrenheit, is known to be favorable for the slow fermentation of beers. The ease of sandstone excavation allowed an extensive network of brewery caves to be mined out in the cliffs lining the Mississippi River Valley. Well-known brews such as Hamm's, Yoerg's, North Star, Schmidt's, and Grain Belt all got their start in Minnesota Saint Peter sandstone caves.
Cellars & Storage
The purity of the quartz in the Saint Peter has made it a desirable commodity for many industrial applications.
Glass ManufacturingThe Ford Motor Company operated an automobile assembly plant in Saint Paul for almost 100 years. The glass used in vehicles they produced was created from local silica excavated from miles of tunnels in the nearby sandstone. Though the plant has now been demolished and the area rezoned, the longevity of this plant is likely due to the unique geographic and geologic location.
Mushroom CultivationThe cool, dark, and damp environment of the tunnels left behind from silica mining operations proved to be a prime habitat for cultivating mushrooms. A stretch of river in the south side of Saint Paul was once known as Mushroom Valley. Here, mushrooms were cultivated in horse manure on the ground (later in more sterile trays) for nearly a century. The last remaining mushroom growers were forcibly evicted from the caves about 30 years ago, but some wooden trays are alleged to remain as a tribute, slowly rotting away in the dark, forgotten space.
Utility TunnelsCity designers and engineers quickly realized another convenient use for the Saint Peter sandstone. The ease of excavation made it possible for a complex labyrinth of utility tunnels to be excavated beneath the city of Saint Paul. The thickness of the sandstone has allowed a literal maze of tunnels to be constructed by different companies. Hundreds of miles of storm and sanitary sewers have been etched out, often by using a pressurized water spray to blast the sandrock away. Other tunnels carrying electricity, gas, water, telephone, ethernet, and pressurized steam route beneath downtown city streets in a dizzying array.
EntertainmentIn the early 20th century a couple of the sandstone caves near Mushroom Valley took on a new role - that of an underground bar. The Wabasha Street Caves, formerly the Castle Royal night club, is still in operation today as a privately-owned commercial cave venue. Another club known as Mystic Caverns operated briefly in the 1930s. It featured live music, a hall of mirrors, live entertainment, a unique bar and dining area, and a comfortable temperature year-round.
ShelterDuring World War II and the Cold War of the 1950s and 1960s, the caves were evaluated for their suitability as a shelter for bombing attacks and nuclear fallout. In the 1967 Fallout Shelter Location manual for St. Paul, five caves are listed as 'active shelters' stocked and ready for use. The caves are no longer suitable for this purpose.
FrackingDue to their fine size and extreme roundness, sands from the Saint Peter are often implemented in the petroleum extraction technique known as hydro-fracking, or simply fracking. The controversial practice involves injecting a pressurized proprietary fluid containing dispersants into target rock units in order to create new fractures and release additional petroleum. Saint Peter sands are included with the fluid to help brace open these fractures and prevent them from resealing.
Though trespassing in them is technically illegal, the vacant caves and tunnels in the sandstone of the Twin Cities remain an alluring space for exploration. The vast networks of passages and immense vaulted ceilings entice the imagination and beckon all with the faintest interest in adventure.Lights and paint adorn the walls as a Wolfcat hunts for echoes of the past.
Tragically, people have died in the caves over the years, and this has increased their stigma. Ironically, many of the measures that the City of St. Paul has taken to block access to these caves have merely made them more dangerous. Several of the caves are filled with rubble and flammable debris from demolished structures, while others have had all of their vents and openings sealed with concrete and rebar.
A 30-foot high cave passage almost completely filled with concrete, dirt, metal, and other debris.
In a matter of weeks, explorers are able to dig new openings through the soft sandstone and regain access. However, because of sealing efforts, the natural air flow in the cave is significantly reduced. Couple this with the instinctual tendency of people to start fires for warmth or out of boredom and you have a recipe for disaster. Anyone who chooses to explore underground locations and abandoned structures should use great care and caution. Make contact with the local urban exploration groups for tips and advice on how to safely enjoy these extraordinary spaces and appreciate the Saintliest of All Sands...
~Geologic and hydrologic aspects of tunneling in the Twin Cities area, Minnesota, United States Geological Survey (1979)
~Brick, Greg A. Subterranean Twin Cities. Minneapolis: University of Minnesota Press, 2009.
~Thiel, George A. "Sedimentary and petrographic analysis of the St. Peter sandstone." Geological Society of America Bulletin 46.4 (1935): 559-614.
~Many of the provided archive photos are from the Minnesota Historical Society