The outwash channel is symbolized by Qgo, which means Q uaternary g lacial o utw

Question

The outwash channel is symbolized by Qgo, which means Quaternary glacial outwash (i.e. gravel and sand from glacial outwash processes). The areas east and west of the South Tacoma outwash channel are symbolized with Qgt, which means Quaternary glacial till. The symbols for each colored area appear as you roll the cursor over them.

Describe glacial till with respect to grain sizes present and other general attributes. (This may require independent research on your part beyond your textbook.)

Contrast the texture of these two types of deposits with respect to the following questions.

Which has more fine sediment?

Which may be more compacted, having been over-run and compacted by ice?

Which is looser and less compact as a result of being deposited by running water?

Based on grain sizes present and degree of compaction which should have greater permeability? (Think if you had a jar of each sediment type and then poured water into them. Which would water penetrate more easily?)

5) When Pierce County receives greater than average precipitation for two years or more, shallow groundwater flooding can become an issue. In which areas is a rising water table likely to cause the flooding, in Qgo or Qgt?

In which areas, Qgo or Qgt, is slow infiltration likely to cause flooding?

Pierce geol pierce units remerton Wother values GIC Ecf Ecf2pgi Ecf23) Evcin MOian MO -Wa Mvaffp) Mvc CEvba OE CEvt Oapfwh) Spanaway Pierce Oclp) Janglewilde-hompson Place 8 BYelm gpfst) Rainier Qoa(sk) ei Eatonville " La Grande Google earth Bogucki Island Imagery Date: 4/19/2015 47003'16.52" N 122019'04.84" W elev 536 ft eyea mi

Explanation / Answer

Glacial till is unsorted sediment deposited directly by glacial ice. Glacial till is a soft rock identified by large angular rock fragments on the surface and within the soil.

Quaternary glacial outwash is the sorted and stratified sediments deposited by meltwater flowing away from the glacier. Outwash deposits are layered in beds because the flow of water that moved the material varied in gradient, volume, velocity, and direction. As a meltwater stream carried the rock materials along, it sorted them by size. As stream velocity decreased, heavier gravels and cobbles were deposited before fine sands, silts, and clays, which were deposited farther downstream. Typical Quaternary outwash in Illinois consists of multilayered beds of sands and gravels and some silts. These beds look much like modern stream deposits in some places. Outwash tends to be coarser and less weathered than stream sediment (alluvium), which is generally finer than medium sand and contains variable amounts of weathered rock debris.

Glacial till has finer sediments.

Glacial till is more compacted due to variation in grainsize.

Glacial outwash is looser and less compact as a result of being deposited by running water.

The primary porosity of unconsolidated sediments is determined by the shape of the grains and the range of grain sizes present. In poorly sorted sediments, those with a larger range of grain sizes, the finer grains tend to fill the spaces between the larger grains, resulting in lower porosity. Primary porosity can range from less than one percent in crystalline rocks like granite to over 55% in some soils. The porosity of some rock is increased through fractures or solution of the material itself. This is known as secondary porosity.

Permeability is a measure of the ease with which fluids will flow though a porous rock, sediment, or soil . Just as with porosity, the packing, shape, and sorting of granular materials control their permeability. Although a rock may be highly porous, if the voids are not interconnected, then fluids within the closed, isolated pores cannot move. The degree to which pores within the material are interconnected is known as effective porosity. Rocks such as pumice and shale can have high porosity, yet can be nearly impermeable due to the poorly interconnected voids. In contrast, well-sorted sandstone closely replicates the example of a box of marbles cited above. The rounded sand grains provide ample, unrestricted void spaces that are free from smaller grains and are very well linked. Consequently, sandstones of this type have both high porosity and high permeability.

A rising water table likely to cause the flooding, in Qgo.

slow infiltration likely to cause flooding in Qgt.

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