Laboratory Manual for Introductory Geology 4e

EXERCISE 17.5
Name:
Course:
Measuring Sea-Level (and Lake-Level) Change (continued)
Section:
Date:
Lake Erie, Ohio
The shorelines of many lakes that formed shortly after the retreat of Pleistocene glaciers from North America have changed
markedly in the past few thousand years. Some glacial lakes have shrunk to a fraction of their former size (such as Glacial
Lake Bonneville, which is now Great Salt Lake in Utah) or disappeared entirely (Glacial Lake Hitchcock in Massachusetts).
The Great Lakes, however, have adjusted to post-glacial conditions, and their shorelines reveal those changes. FIGURE 17.7
is a topographic map of an area in Ohio just south of Lake Erie.
(e) Examine the spacing of the contour lines on the map. What do they suggest about the evolution of Lake Erie?
(f) Draw a profile along line A–B (using the graph paper provided at the end of this chapter) and then compare it with
the profile you drew for question (b). What features are probably represented by the ridges? What features are
probably represented by the gently sloping areas between the ridges?
(g) Given your answers to question (f), how are Sugar, Chestnut, and Butternut ridges related to the post-glacial
history of Lake Erie?
(h) Label the previous shoreline position(s).
(i) The current elevation of Lake Erie is 174 feet. How much has the lake level changed?
(j) Was the change continuous or did it take place in sporadic episodes? Explain.
(k) Assuming that the retreat of continental glaciers took place about 10,000 years ago, calculate the rate at which
lake level would have dropped if the change had been continuous.
(l) If the change was episodic, suggest a way to estimate the relative amount of time associated with each “still-stand”
of lake level. What assumptions must you make?
442 CHAPTER 17 SHORELINE LANDSCAPES