Glacial geology
Although limited to Greenland and Antarctica today, extensive ice sheet glaciers covered many continental areas, including much of the state of Minnesota, in the recent geologic past. These glaciers scraped, scoured, moved, and deposited vast quantities of sediment over this time.
The surficial topography and geology of Apple Valley, Minnesota was influenced by the Superior and Des Moines glacial lobes during the Wisconsin-age glaciation between about 75,000 and 12,000 years ago. Apple Valley sits right where the edge of the ice sheet existed.
Two forces work together to cause erosion associated with glaciers—plucking and abrasion. Plucking occurs when meltwater flowing beneath the glacier seeps into cracks in the bedrock. The water freezes in the cracks. As the glacier moves pieces of rock are ripped from the bedrock and then dragged along the surface. Many of these rocks stay frozen and locked within the glacial ice as it continues its flow. Abrasion is caused by all the grit embedded in the ice. As the glacial ice flows, this embedded sediment scrapes and scours the bedrock below the glacier.
Unlike running water which can only transport a certain volume (capacity) of solid sediment based on the volume of water flowing and a certain size (competence) of sediment based on velocity of flow, ice can transport a nearly limitless amount of sediment and material of any size.
All the sediments eroded and transported as glaciers flow are ultimately deposited either beneath the glacier or at its edges. Glacial deposits in any form are called glacial drift.
Two different types of glacial drift exist—till and stratified drift. Glacial till is sediment deposited directly by ice as it melts. As the ice melts it drops all the sediment it is carrying. Since ice carries sediments of any size, till deposits are very poorly sorted mixtures of boulders, gravels, sands, silts, and clays.
One type of till deposit is called a moraine. End moraines are large ridges of till that form at the end of the glacier. Some continental ice sheet end moraines extend hundreds of miles and may be over one hundred feet high. In Apple Valley, areas covered by moraine deposits often have irregular, hilly topography.
The second form of glacial drift is called stratified drift. Stratified drift is deposited by running meltwater from the glacier. Stratified drift deposits are better sorted than till and dominated by gravels and coarse sands. The larger, heavier boulders are left behind in the moraines while the smaller silts and clays are carried farther away from the glacier-dominated area.
Outwash plains are blankets of stratified drift carried and deposited by meltwater beyond the area covered by glacial ice. In Apple Valley, outwash plain deposits form flatter, more uniform topography.
Bedrock and groundwater geology
Solid sedimentary rocks deposited during the early Paleozoic Era sit beneath the glacial sediments in Apple Valley. These rocks consist of carbonate rocks like limestone (CaCO3) and dolostone (CaMg(CO3)2) along with other sedimentary rocks including sandstone and shale. These rocks were deposited during the late Cambrian and early Ordovician Periods (around 480-500 million years ago) when this region was covered by a shallow, tropical sea. Although these rocks are deeply buried beneath the surface, they are very important for the city.
The city of Apple Valley draws most of its groundwater from the Prairie Du Chien-Jordan and Jordan aquifers within these Cambrian and Ordovician rocks. The Prairie Du Chien aquifer consists of limestone while the Jordan aquifer is sandstone. These wells tap into groundwater ranging from 487 to 516 feet below the surface. In 2021, the city extracted over 2.5 billion gallons of water from these aquifers. Groundwater extracted from these aquifers has a hardness of 17 grains per gallon.
Water hardness is mostly due to calcium and magnesium in the water. As water moves through rocks it dissolves some of these minerals and carries ions of calcium and magnesium away with water. Excessive hardness can cause problems including damage to pipes and equipment as minerals in the water are re-deposited on surfaces as lime scale.
The USGS classified hardness of water as follows:
- 0-60 mg/L = soft water
- 61-120 mg/L = moderately hard water
- 121-180 mg/L = hard water
- 180+ mg/L = very hard water
1 grain per gallon is equivalent to 17.1 milligrams per liter (mg/L) or 17.1 ppm (parts per million).
Logging requirements:
In order to log this cache, you must send answers to all of the following questions to the cache owner. Do not post your answers or include photos of the signage in your "Found it" log. Please send your answers at the same time as you log your find and include the cache name in your message.
Use information at the coordinates and in the cache description above to help answer these questions.
1. Which part of Apple Valley consists of rolling hills? Is the topography of that area due to glacial moraine deposits or outwash plain deposits? Which part of Apple Valley consists of flatter terrain? Is the topography of that area due to glacial moraine deposits or outwash plain deposits?
2. When did mining of glacial sediments first occur in Apple Valley? How many acres of land in Apple Valley were mined?
3. What sediment sizes make up most of the mined materials in Apple Valley? Are glacial sediments mined in Apple Valley till or stratified drift? Were these mined glacial sediments deposited directly by ice or by running glacial meltwater?
4. Describe how the sizes of sediment in the glacial deposits help determine whether they were deposited by ice or running meltwater.
5. Using information about the hardness of Apple Valley’s groundwater provided in the cache description, calculate its hardness in milligrams per liter (mg/L). Is water drawn from these aquifers in Apple Valley soft, moderately hard, hard, or very hard?
6. What evidence of the hardness of the water do you see at the posted coordinates? Please be specific: describe what you see including any distiguishing characteristics, where you see it, and what is causing it to be there (the specific cause may be easiest to see during summer months).
References:
City of Apple Valley. (2022). 2022 Apple Valley Water Quality Report. Retrieved from https://www.ci.apple-valley.mn.us/DocumentCenter/View/967/2022-Water-Quality-Report?bidId=
Lusardi, B.A.; Dengler, E.L.. (2017). Minnesota at a Glance Quaternary Glacial Geology. Minnesota Geological Survey. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/59427.
Ojakangas, R.W. (2009). Roadside Geology of Minnesota. Mountain Press Publishing Company.
USGS. (2018). Hardness of water. Retrieved from https://www.usgs.gov/special-topics/water-science-school/science/hardness-water#:~:text=Hardness%20is%20caused%20by%20compounds,a%20variety%20of%20other%20metals.