Lesson 11: Great Valley
If you look at a geologic map of California you'll notice that three of the state's largest geomorphic provinces—the Coast Ranges, the Great Valley, and the Sierra Nevada—lie parallel to one another and are elongate in a north-northwest direction. As you might imagine, this is no coincidence. The basic geologic structure of each of these regions was established during the Mesozoic Era when California's geology was dominated by a north-northwest trending subduction zone that ran the length of the state. Subduction zones formed along continental margins are commonly marked by four parallel features: an ocean trench, an accretionary wedge, a forearc basin, and a volcanic arc. The trench that once lay off the central California coast is long gone, but the other three features are preserved today by the Coast Ranges, the Great Valley, and the Sierra Nevada batholith, respectively. We've already explored the Sierra Nevada and this week will look at the Great Valley and trace its transition from a marine forearc basin, through a period of nonmarine deposition, to the formation of modern soils that make it a rich agricultural landscape (Sacramento Valley rice fields, right) today.
In chapter 11 Harden moves chronologically backwards through the geologic history of the Central Valley. She begins by describing the young glacial outwash and floodplain deposits that blanket the modern valley floor and the development of the soils that make these deposits so fertile. She goes on to recount the history of marine and non-marine deposition that has filled the valley with up to 20,000 meters of sediment and describe the brief episode of volcanism that occurred when andesite and rhyolite magmas pushed up through these sediments to build the Sutter Buttes 1.3-1.6 Ma. Harden concludes her review by examining the valley's "basement"—the Coast Range ophiolite—where its exposed along the valley's western margin. Finally, this week's exercise introduces you to some of the concerns that geologists need to weigh when they seek to find a site for a new landfill—as geologists working for the city of San Francisco are doing in the Great Valley today.
As you read about California's Great Valley be sure to take careful notes on the topics covered by the learning objectives below. As during previous weeks, you'll find that writing out key facts in your own words or making neatly labeled drawings will help you better understand the significance of what you've read and spot any gaps in your knowledge. Don't hesitate to post any questions you have to the Discussion Board so that your classmates or I can help you figure them out. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you want to refer back to it for future assignments. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Briefly describe where the young (Pleistocene and Holocene) sedimentary deposits that cover the surface of the Great Valley came from, and why the oldest such deposits are now exposed in river terraces on the eastern side of the valley.
- Explain how the A and B horizons of a soil differ, and suggest how you might estimate the relative ages of a group of soils based on a variation in the development of these horizons.
- Describe how irrigation can actually lead to a worsening of soil fertility in the Great Valley due salinization.
- Contrast the types of volcanic rocks that form the inner core and outer rampart of the Sutter Buttes, and indicate if the eruptions of andesite and rhyolite lavas that built each of these features was explosive or effusive.
- Use relative dating principles, such as superposition and cross-cutting relationships, to infer the sequence of geologic events that occurred in the Great Valley as shown by cross-sections such as Figures 11-3, 11-15, and 11-22.
- Outline the major events that shaped the geologic history of the Great Valley—from the development of the Coast Range Ophiolite to the deposition of modern alluvial sediments and glacial outwash—and tell the approximate age or age range for each of these major events.
Reading and Browsing Assignment
- Read Chapter 11, being sure to focus on the topics outlined in the learning objectives above.
- To learn a little more about how soils develop, browse through this brief introduction to soil horizons and check out this Shockwave animation showing how soils are gradually modified as eluviation washes materials out of their A and E horizons and illuviation washes these materials into their B horizons.
- To get a little different perspective on the Sutter Buttes check of their description and that of the surrounding Great Valley in the U.S. Geological Survey's Tapestry of Time and Terrain.
- Finally, check out this brief photographic "tour" of part of the Great Valley by Andrew Alden on the About.com:geology site. Clicking on the text next to each picture will bring up a larger image as well as a short descriptive paragraph.
Exercise 11: Landfill Siting (Due by 9:00 AM on 28-Mar-2011)
In recent years people from the Great Valley (Sacramento, in particular) have begun trucking some of their trash over the Sierra Nevada to a landfill in Nevada! Similarly, those of us in Siskiyou County have been trucking our trash over the Siskiyou Summit to Josephine County, Oregon. If this seems crazy and you think there must be plenty of closer places to build a new landfill you may not know how difficult it is to actually find a suitable landfill site. To get a little better appreciation of the problem, please load up your Hazard City CD and work through version 1 of the Landfill Siting exercise. There are a number of things to check for at each proposed site, so I suggest that you print the worksheet provided on the report page and just fill it in as you go. When you're done, go to the "Assignments, Tests, and Surveys" tool in Etudes and report you findings by answering the questions in Exercise 11. Note that I have combined questions 1 and 9 into a single question and that, because there only eight questions, each is worth 1.25 points.
Quiz 11: Great Valley (Due by 9:00 AM on 28-Mar-2011.)
After you feel you have met the learning outcomes outlined above please complete Quiz 11 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about the geology of the Central Valley and each is worth one point. If you can answer all of them correctly it means that you know your way around the Central Valley pretty well and are ready to start learning about California's Coast Ranges next week. Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.
