For a printable copy (pdf) of this earthcache, click here.
What is a surficial geology map anyway?
A surficial geology map shows the areal distribution of all the loose materials (called “surficial materials”), such as till, sand and gravel, or clay which overly solid bedrock. These materials are not soils; they lie deeper, between the soil zone and underlying bedrock. Most of the surficial materials found here in Illinois are deposits formed by glacial processes during the last stage of continental glaciation from about 24,000 to 12,500 years ago. The remainder of surficial deposits are the products of more recent processes, such as river flood plains, or are attributed to human activity, such as road fill or other land-modifying features.
How are materials grouped?
Geologists group surficial materials as “lithostratigraphic units.” Each unit was deposited during a specific time period and has its own mix of materials (sand, gravel, till, etc.). Every unit is given a name that is used on maps/reports throughout the state or country. For example, the “Grayslake Peat” is recent (less than ~ 14,700 years ago) “decomposed wetland vegetation and sediment in depressions and on slopes associated with discharge of groundwater.” All materials that were formed during the same time period and follow the description of Grayslake Peat are all given that name (even if it’s not located in the town of Grayslake).
How are these materials mapped?
Geologists rely on various data sources and field investigations to create their map. Data for the Surficial Geology of Elgin Quadrangle map came from: sampled borings at key locations, descriptions of glacial sediment at abandoned quarries, sampled borings from construction and proposed mine activities, descriptions from drilling water wells and test wells, field investigations of rock outcrops within the area, and past studies. Geologists use this data and their locations to create a cross section, a vertical slice into the earth (see example below) between points on the map. Once complete, boundaries are drawn between map units on the areal map. Geologists also rely on soil maps and topography to help draw these lines where data doesn’t exist.
So, how do I read this map?
Each unique color on the map represents a different lithostratigraphic unit. The map displays them as if all the soil, water, and infrastructure were removed and you were looking down from an airplane. Within the colored areas are letters that indicate the unit in the legend. Normally there is a capital letter followed by one or more lowercase letters. The age of deposition is represented by the capital letter at the beginning of the label (for example, O = Ordovician). The remaining lowercase letters refer to the lithographic unit (for example, gp = Grayslake Peat). On this map, since all the surficial materials were deposited in the Quaternary Period, the capital letters are left off the map (you will see some from other periods labeled on the cross-sections).
Additionally, the locations where borings or observations were made about the surficial materials are indicated on the map as points of different colors. The colors represent the type of observation made. The lines that connect these points make up the cross-sections that were created during map construction.
The lithostratigraphic units are mapped on top of a base map. Here, the base map is a USGS topographic map (also called quadrangle) and shows roads, contours, some buildings, creeks and ponds, section lines and numbers, and so on.
What are these maps used for?
Surficial geology maps may be used to: aid in search for water aquifers, locate deposits of sand and gravel for aggregate uses such as construction, locate deposits of clay used in brickmaking or pottery, address suitable placement for environmental issues, such as landfills and underground storage tanks, evaluate possible extent of contamination spills, and placement of new roads and other infrastructure.
Questions to answer:
- Find yourself on the map (see clip in gallery or for the full map, click here). If you’re not good at reading maps, take it slow. Find I-90 and Rt. 31 interchange. Follow 31 north until you see Jelkes Creek. Then follow Bankosky Road to the west. Keep in mind that we are in a restored quarry (which is indicated on the map by diagonal hash marks). What lithostratigraphic unit are we in (prior to mining)?
- Point 457 on the cross-section, which is just northeast of here, indicates that the lithostratigraphic unit you named in Q1 is about 65 feet deep. Use the difference in elevation from the earthcache location to the lowest level of the former pit (parking lot) to estimate the pit’s depth. How does this compare to the depth of the surface unit at point 457? Do you think the area was completely mined out? Why or why not?
- On your way to or from the earthcache location, stop by the waypoint 1 below (N42 05.117, W88 18.626). There is a deer trail (or perhaps a geotrail) where the earth is exposed. Describe the types of rocks you see here. What size(s) are they? Are they rounded or jagged? Are there many or few compared to the surrounding matrix? Do they vary or similar in color? Is this what you expect to see based on the description of the unit in Q1? Why or why not?
Please email the answers to me to log your find. BTW, just a note on my philosophy of earthcache questions: It is my hope that you learned something (perhaps interesting) and made some observations about the world around you when finding this earthcache. Do not stress about getting “correct” answers. A quote from the book Rising from the Plains by John McPhee sums it up nicely: “Say not ‘This is the truth’, but ‘So it seems to me to be, as I now see the things I think I see.’”
Sources
Surficial Geology of Elgin Quadrangle, Kane and Cook Counties, Illinois, 2007, Illinois State Geological Survey.
Reading Detailed Surficial Geology Maps, Maine Geological Survey, accessed October 8, 2013.
Please see Image Gallery for cross-section, map, and legend. The printable PDF (at top) also includes these items.