University of BASS BRIGADE
Course: Earth Science 101 - Glacial Erratics Credits: 1 Geocaching Find
Instructor: Dr. BassBrigade, NPG (Non-Professional Geologist)
Hello and welcome to my class! Today’s adventure is a two-part assignment. We will begin in the classroom setting where you will learn about glaciers, how their advancement shaped our land, the types of rock that are native and non-native to present day Illinois, and how to identify the probable home base of a select group of non-native rock types. From there you will be required to perform a field study, which will allow you to determine if the proposed EarthCache is infact a true glacial erratic. Once you have completed your Field Journal, submit your research to us by email or message via your geocaching app or through geocaching.com. Please note that attaching a picture of you or a personal item in the field is a requirement to receive credit for this assignment. Oh, and one last thing, watch your step out there.. The terrain is a little "rocky".
What is a glacier?
I'm glad you asked! The term glacier comes from the French word glace (glah-say) which means ice. A glacier is defined as a slowly moving river of ice, however not all masses of ice are considered glaciers. To qualify as a glacier the ice must be at minimum .006 of a square mile in size, and over 164 feet thick. Glaciers form in places where snow accumulates over long periods of time. As the snow becomes deeper, the weight and pressure turns the bottom layer of the snow into ice and water. This high pressure along with gravity causes the glacier to expand outward. While most glaciers move slowly, some move as fast as 100 feet per day. Historically, glaciers have played a major role in shaping the earth’s landscape through erosion. Water from the bottom layer of the glacier seeps into cracks in the earth, where it freezes and expands, loosening rocks and boulders. These rocks are picked up and dragged along the glacier's path by an action referred to as “plucking”. As this debris is pushed forward it can be carried hundreds of miles, often filling in any low land that it encounters.
Dr. Bass Brigade, I have heard about periods of time referred to as the Ice Ages. Can you elaborate?
Absolutely! An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental polar ice sheets and alpine glaciers. Research shows that the Earth has experienced 5 major Ice Ages.
- The Huronian Glaciation, 2.4 to 2.1 billion years ago
- The Cryogenian Glaciation, 720 to 635 million years ago
- The Andean-Saharan Glaciation, 450 to 420 million years ago
- The Late Paleozoic Ice Age, 335 to 260 million years ago, and the
- The Quaternary Glaciation 2.7 million years ago to present
It's important to note that major Ice Ages fluctuate, with smaller ice ages known as Galcials being followed by warmer periods known as interglacials. In the last million or so years, huge glacial sheets have appeared in cycles of 100,000 years. They grow and expand outward for around 90,000 years, where they then take around 10,000 years to mostly recede during the warmer periods. Research indicates that while our current climate is rather warm at times, the Earth is actually cooling, indicating that the current cycle is ending. In using the 90/10 model against the theory that most of the ice covering the earth during our recent ice age disappeared more than 11,000 years ago, some speculate that the earth should be headed for another glacial period. However, in recent decades scientists have identified factors in the Earth’s orbit that influence the cycles of Glacials and InterGlacials. The irregularities in these parameters could cause delays in the Ice Age cycle. In addition, there are other manufactured disruptions in the environment such as the amount of carbon-dioxide that is being expelled into the Earth's atmosphere.
What Rocks are native to Illinois?
Great question! Most of the rocks native to Illinois are sedimentary rocks. Some are made of small pieces of shells, plant and animal remains, and also weathered fragments of other rocks. These rocks have been moved by, of course, glaciers, but also rivers, waves, and winds. Clastic sedimentary rocks are classified by their grain size or texture. Examples include clay, gravel, sand, and silt. Additionally there are types of rocks that are formed by chemical precipitation. Examples of these are dolomite and limestone. Knowing that these 2 form most of the bedrock in this area, let's break the two down a bit further.
DOLOMITE
Dolomite, also referred to as Dolostone and Dolomite Rock by weight, contains more than 50 percent of the mineral Dolomite. It's appearance is pearly or glassy, and forms when limestone undergoes a chemical change by coming in contact with Magnesium-rich groundwater. Dolomite is usually very similar to limestone, found in the colors of yellowish to brownish, though it is slightly harder.
LIMESTONE
Limestone is composed primarily of calcite, a calcium carbonate mineral with a chemical composition of CaCO3. It usually forms in clear, calm, warm, shallow marine waters. Limestone is usually a biological sedimentary rock, forming from the accumulation of shell, coral, algal, fecal, and other organic debris. It can also form by chemical sedimentary processes, such as the precipitation of calcium carbonate from lake or ocean water. The color of Limestone ranges from white, yellow, and gray to blue, beige, and cream. When compared to dolomite, it is slightly softer and tends to crumble easier.
So, If these are non-native erratics, where did they come from?
Another good one! To understand how these erratics found rest at this location, first we will need to travel back 2.6 million years to the Pleistocene Epoch. By this time the continents as we know them today had already moved to their current locations, and large sheets of ice had begun forming at the North Pole, expanding downward over Canada. As the ice pushed south across North America it carried massive amounts of rock and earth hundreds of miles. Some parts of Illinois were covered in ice as thick as 2,000 feet, while others were covered in as little as 700 feet. At its highest point the ice would have blanketed everything in northern Illinois including the skyscrapers in current-day Chicago.
During this time the average temperature across each season ranged between 7 to 13 degrees lower than what we experience today. This made for mild summers, which often prohibited the snow and ice from completely melting prior to the following winter. As the years progressed, the fluctuation of the temperature did allow for the ice to recede and advance multiple times. This process and ongoing growth continued until the climate became more temperate, increasing the melting rate over the rate of expansion. Following the melting of the last glacier around 11,000 years ago, the landscape took on a new appearance when compared to its pre-glacial form. A vast majority of the hills and valleys were completely filled in, new ones had formed, and glacial drift covered much of the land. Drainageways for meltwater from the north, northwest, and northeast were relocated, also merging in Illinois. Once the melting was complete, the land could once again prosper.
So where did this proposed erratic in Illinois begin their journey? Given what we have learned about the outward expansion on the glaciers that have covered Kankakee County in Illinois, we can compare intrusive rocks to their base locations and form an educated guess as to where they originated.
- Rocks transplanted from widely across Canada are:
GRANITE
Granite is made mainly of quartz, feldspar, biotite and muscovite; it sometimes also contains hornblende, augite, magnetite or zircon. Most of quartz’s color comes from feldspar, which yields white, light gray, yellowish or pink tones. Biotite and hornblende produce black specks, and muscovite imparts a silvery or brownish color. The Grain of granite ranges from coarse-grained to very coarse-grained. It has a distinctive mottled appearance from the intermingling of all its component minerals. Please don’t attempt to while determining if the proposed EarthCache is granite, but the hardness is also a key identifier, where scratching the surface will generally not lead to visible scarring. Additionally granite can be found in a wide array of colors
GNEISS
Gneiss ranges from coarse-grained to very coarse-grained. It has a distinctive mottled appearance from the intermingling of all its component minerals.The texture is considered Foliated, with foliation on a scale of a cm or more, and the color is generally alternating lighter and darker sub-parallel discontinuous bands.
TILLITE
Tillite is a sedimentary rock that consists of consolidated masses of unweathered blocks and glacial till rock material deposited by glacial ice in a rock flour matrix or paste of unweathered rock. The matrix, which comprises a large percentage of the rock, usually is dark gray to greenish black in colour and consists of angular quartz and feldspar grains and rock fragments in a very fine-grained paste.
- Rocks carried in that are specific to Ontario Canada are light gray and white Quartzine boulders with rounded pebbles of red Jasper.
- Rocks found with flecks of copper match those found in the Upper Peninsula of Michigan
- Purplish pieces of Quartzite are shown to have traveled from Wisconsin
Can you tell us more about the glacial and interglacial episodes of the Pleistocene Epoch?
I sure can! I have actually prepared a slide covering both the Pleistocene Epoch and the Holocene. Let's pause for a moment while you do a self study prior to us moving on..
All right class, I think it's time to cut you loose so that you can head out into the field and help me determine if the proposed EarthCache is infact a glacial erratic! You will find your Field Journal below. I have also provided additional waypoints for parking, and the required virtual stages.
Welcome to Riverfront Park in Bourbonnais Illinois. This virtual earthcache requires around .25 miles of walking on paved trails, a moderately sloped grassy area, and a manageable decent into a ravine. Permission for the placement of this earthcache was provided by the Village of Bourbonnais. Please watch your step as you head into the ravine near the provided coordinates. There is also a disc golf course who’s flight path is nearby, so keep an eye out for a rogue disc if anyone is playing during your visit! Park hours are from sunup to sun down.
Requirements/Field Journal
- First follow the footpath west until you cross the bedrock ravine by footbridge, then safely make your way down to the base of the bedrock. Examine the large walls of stone, feeling the texture and hardness of the rock. Using what you have learned, what type of rock largely makes up the ravine walls?
- Above the shown signs of undercutting from years of water run-off, study the walls closely for aged scrapes indicating possible glacial activity. Given that the river is to your south, what direction do the surface level and deeper striations run?
- Next, safely make your way out of the ravine, back across the footbridge, and over to the proposed erratic. Examine the texture, hardness, color, and comparison to the bedrock. Using what you have learned regarding the native and non native rock to Illinois, what type of rock would you classify this boulder as?
- Now that you have identified the type of rock, you can also make an educated decision on its basecamp. Using what you have learned, where did this boulder more than likely come from?
- And finally, the most important question, is the proposed EarthCache a glacial erratic?
- When logging your find don’t forget to attach a photo of you or a personal item with some part of the location’s landscape visible in the background. Thank you!