A FIELD-BASED EXPLORATORY ACTIVITY TO INTRODUCE SEDIMENTARY ROCKS
Anne Egger Stanford University
Students are introduced to sedimentary rocks through field observations and measurements of sediments in different environments. This allows them to proceed from processes to products and engages them through field work in the local area.
GSA Poster (Acrobat (PDF) PRIVATE FILE 15.6MB Nov5 04)
1. sedimentary processes including transport and deposition of sediments, sorting, and rounding
2. sedimentary rock types, especially clastic rocks
1. how to make quantifiable observations of sediments and sedimentary rocks: cobble counting to determine source, sieving samples to determine sorting
2. sketching outcrops
Higher Order Thinking Skills:
1. using deductive reasoning to determine the source, transport, and depositional environment of a sediment and/or sedimentary rock from observations made
1. gaining comfort with the ability to learn/perform science
undergraduate entry level
teacher professional development (K-5)
Students must have learned about weathering and erosion, including knowledge of a few key minerals: quartz, feldspar, clay, etc. All sedimentary concepts can be taught in the field in the context of this exercise: grain size, rounding, sorting, etc. Alternatively, students can be given readings about stream morphology ahead of time so that they have a sense of their location within the bigger sedimentary picture.
Role of Activity in a Course:
My intro-level geology course is primarily project and activity-based, with several small field trips integrated into it. This exercise would also work well as a stand-alone lab or field trip, however, as long as post-trip classroom time is factored into the activity.
Data, Tools and LogisticsRequired Tools:
I use sieve sets, which can be purchased from Ward's for about $25. Corers can be improvised with PVC pipes and dowels. Other materials include string, hand lenses, shovels, buckets, rulers, all of which are readily and cheaply available. Nothing requires special training or significant amounts of money or time to assemble.
Although this activity can be modified to fit a wide range of locations, seasonal weather and stream variations can be a challenge. A back-up plan for the stream in flood would be a good idea.
I would like my students to be able to connect the characteristics of sedimentary deposits to the processes that caue them. THey should understand that the amount of energy relates to the grain size, that the transport distance affects the grain size and sorting and rounding, etc. I don't care as much about the names of things.
I would also like to see that their own perception of their ability to learn and do science has increased. Through making repeated abservations and drawing conclusions based on them, they clearly gain confidence in their abilities.
I don't feel that I have been fully successful in my assessment, though I am confident that learning has taken place. Students are much more comfortable with sedimentary concepts than metamorphic or igneous, as reflected in descriptions of these processes they write in their final papers.
Anecdotally, students also really enjoy this lab. They particularly like learning something new about a place they are already familiar with because it is nearby.
As an introduction to sedimentary rocks and processes, students are guided through a two-stage field laboratory in a local canyon, the Farmington Glade.
In the first stage, students examine sediments in a dry wash. Students describe the sorting, rounding, and composition of grains, and relate these properties to the source, transport, and deposit of the sediment.
In the second stage, the students make similar observations about the Paleocene Ojo Alamo Formation, a fluvial sandstone that forms the canyon walls. Students perform a cobble count, sketch cross-beds and channels in the outcrop, and discover abundant petrified wood within the formation and discuss reasons for its presence. A follow-up discussion in the field helps students use this information to deduce the source, transport, deposit, and additional information about the formation.
I have recently moved from San Juan College to Stanford University and modified the activity to accomodate a larger class size, few rock outcrops, and few non-engineered streams. For the modified version, students went out to four different locations along a hypothetical longitudinal stream profile: the headwaters, where landslides occur, the upper alluvial fan, the lower alluvial fan, and the baylands. Wach group made observations, collected samples, and brought the information back to their discussion section, where they shared information with other people. The TAs then helped the students connect the different sediment types to different rock types.