By Bob Feurer
Glacial outwash gravel and alluvial gravel are created by different mechanisms. Use your Earth Science text or other resources to define each in the space below.
- Glacial till (outwash)
- Alluvial gravel
Materials need to complete this lab:
Stackable screen sieves with mesh nos. 6, 20, 40, 100. Newspaper, white glue, large note cards or oak tag poster board squares at least 4 x 6 inches, samples of alluvial gravel and glacial outwash, Ziploc sandwich bags or similar transparent containers, triple beam or electronic balance.
Analyzing the Samples
Obtain a sample of each of the unseparated materials, spread them on a newspaper at your table and make several pertinent observations of each before separation. Record them in the appropriate areas below.
- Glacial Outwash Gravel
- Alluvial Gravel
Determining Screen Size
Before beginning screen separation of the two samples, determine the screen sizes of each of the separate pans in the stackable screen sieves. You may need to use a dissecting microscope or magnifying glass with your metric ruler to determine these!
- Screen #1
- Screen #2
- Screen #3
Screening the Samples
Select one sample (either outwash or alluvial gravel) and place a sample of about 250 ml volume in the top tray of the separator stack. (The glacial outwash sample may need to be ground up briefly with a mortar and pestle or other means to break up the masses of small particles so they will pass through the screens.) Shake this sample vigorously until it appears the sample is completely separated.
Remove the contents of each of the trays and place it in separate, labeled Ziploc sandwich bags. Determine the mass of each separated sample and record below:
Repeat the procedure on the other sample--separating, bagging and mass determination. Record your mass results below for the contents of each tray:
Now, contrast and compare the separated elements of each of the samples in the appropriate columns below:
Optional activity: Use two index cards and glue a representative rock specimen of each type found in the top tray of the screen sieve. How do the two samples compare?
Discussion and Conclusion Discuss below the ideas you formulated from your observations of the two samples. Use proper English, good spelling, punctuation, etc. This is half of the grade for this activity so do your best work!