Geol 3000 Name___________________________ Geologic Maps Score ________/12 Lab 8A: Surficial Geology of Pope County, Minnesota Refer to the Pope County Atlas (C-15, Part A, Plate 3) to answer the following questions. 1) What is the age difference between Holocene and Pleistocene sediments? _____________________ _______________________________________________________________________________ 2) What are the three main geologic Pleistocene map unit groups in Pope County from oldest to youngest? _______________________-_______________________- _______________________ 3) The thick blue lines on the map represent the outer margin of the ice lobe at a particular stand-still stage (see Fig. 3). Goose River Group sediments tend to be confined inside (SW) of what number ice margin? ___________________________________ 4) What ice margin position developed an earthen dam (moraine) of till which created Glacial Lake Benson as the ice retreated? _________________ 5) Note that in the correlation of map units diagram, the units are grouped by the environment in which the sediments were deposited – glacial sediments (till), river sediments (outwash), and lake sediments. For each of these groups, describe their most common attributes in terms of sediment size, sorting, and bedding. Understand that the term “loam” indicates a mix of sand, silt and clay; if it is very rich in the clay component it is termed clay loam; if it also contains gravel, pebble, and cobble sized particles, the modifiers “gravelly”,” pebbly”, and “cobbly”, are often used. Glacial sediments Sediment sizes: ___________________________________ Sorting: _________________________________________ Bedding: _______________________________________ River sediments Sediment sizes: ___________________________________ Sorting: _________________________________________ Bedding: _______________________________________ Lake sediments Sediment sizes: ___________________________________ Sorting: _________________________________________ Bedding: _______________________________________ 6) How are tunnel valleys interpreted to form? ______________________________________________ _________________________________________________________________________________7) Identify at least 5 lakes that are mantled by tunnel valley walls, describe their orientation (e.g., NE, N, ESE, …), and identify the ice margin number that marks their eastern termination. 1) _____________________________ Orientation: _______, Ice Margin #:______________ 2) _____________________________ Orientation: _______, Ice Margin #:______________ 3) _____________________________ Orientation: _______, Ice Margin #:______________ 4) _____________________________ Orientation: _______, Ice Margin #:______________ 5) _____________________________ Orientation: _______, Ice Margin #:______________ 8) Consider the sediments and landforms in T124N/R37W. See that the tunnel valley mantling Swenoda Lake is eroded out of unit gp and ends at ice margin 3. Then northeast of ice margin 3, the sediment is a mix of ogp, ogr, ogo, and gcc. What depositional environment do you think these mixed sediments represent. ________________________________________________________________ _________________________________________________________________________________ 9) What sediment type do the longer esker landforms tend to develop on and is this consistent with how eskers are thought to form ?___________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ 10) Unit hs is described as holocene “rubble”. How does this type of recent sediment form? _________________________________________________________________________________ _________________________________________________________________________________ 11) Assuming prevailing winds from the west, from what unit(s) was the silt that makes up the loess deposits derived?___________________________________________________________________ 12) Note that the map units are defined by the sediment type, not the interpreted origin of the sediment (the interpretation of each unit is shown in italics at the end of each map unit description). This is very different from the Washington County map. Which way of classifying map units, genetic or non-genetic, do you think is preferable and why. ______________________________________ _______________________________________________________________________________ _______________________________________________________________________________