Mendenhall Glacier Sand Is Not Ground-up Bedrock.

This page is intended to share my 10,000 photos of more than 800 samples from Alaska, Tasmania, Maine and Oregon. I also have samples in my storage unit that I can send you. I need to find a place for all this material. Thanks. Email me.

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“Glacial” Sand and Gravel in the Juneau, Alaska vicinity is not “Ground Up Rock” but Petrified Organic Materials - $10

Near surface sand and gravel on the west shore of Mendenhall Lake and Eagle River Beach near Juneau, Alaska, are not individual particles of glacial sediment but delicate and interconnected networks of magnetite, and silicate materials that could not have moved far from where they formed and could not have been deposited by water and assembled by chance. Mineral grains have crisp, molded, not fractured or abraded, surfaces including pores, spines and ridges which continue onto adjacent grains. Organic structures are plainly visible on the surface and inside the sand, gravel and cobbles. All stages of a continuum between organic material and replacement by silicate minerals and magnetite are found. Magnetic particles in this sand are likely to be the product of bacterial respiration. The magnetite is not pure but is mixed with silica indicating that the silica and the magnetite were deposited together. This is a photo essay arranged by place and sample, for example, Place: Mendenhall Lake and Sample: mend west 1.12.13. 

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Lanky Moss Experiments: sand forms under Rhytideadelphus loreus community - $7

Micro-photos show sand on the west shore of Mendenhall Lake is likely to be a deposit of mineralized peat. A C14 date of 1950 was obtained for organics trapped in magnetite grains as they formed. Silica silica grains form on the growing tips of “Commo Water Moss,” (Fontinalis antipyretica). Since the site was covered by a glacier until about 1940, it appears the sand in the top 100 mm was formed after the glacier retreated and the process is ongoing and current. The sandy beach has a thick cushion of “Lanky Moss,” Rhytideadelphus loreus, growing on the surface. So, if the sand is petrified organic material, and if the process is current and ongoing, then sand under moss must have formed recently and there should be a zone beneath the moss where newly and partially formed grains are common. To test this hypothesis a trowel was used to cut a cylindrical plug of moss and sand from the beach which was then examined from the top down to look for that zone of newly deposited silica and magnetite. This simple experiment confirmed the hypothesis. It revealed sand forming at all layers, growing moss, peat and mineral sand, most noticeably in the peat. Magnetic moss stems with no visible minerals attached were found just above the peaty layer. The community of moss, roots, fungus, blue green algae, bacteria and perhaps diatoms seems to be making the sand. Similar observations of a second patch of Lanky Moss growing on sand at Eagle Beach support this hypothesis.  


Summary and Gallery

Below: The large grain is about 1mm and was collected with a magnet. The dark blue materials are presumed to be magnetite. 1950 is the C14 radiocarbon date of the organic material.

The silica surface is smooth and shiny, but uneven, showing many little opalescent patches that look like pores (A) notice how the organic material goes into, and out of, the grain. Inside it is silica, outside it is organic. B) This is a magnetic particle so this darker bluish material must be the magnetite. However, it doesn’t look like either massive or crystalline magnetite and it does have lots of stems and other organic structures running through it. C) This is a moss leaf. D) A small tubular structure projects from the surface, but it is organic at the tip. There is no way anything this fine survived a glacial rock tumbler.

The “ground up bedrock” theory would suggest this grain was broken from the bedrock in a fractured state. Travel under the glacier would have smoothed the surface… then some time later the moss and roots would have to invade the grain. In this case they would have done that since 1950. How they would grow through a sand grain in such diversity and abundance is a great mystery.

Then after somehow getting inside, the organic material would have to be replaced by minerals. It’s a really complicated story compared to the one where the organic material is there first and the minerals are deposited in them later.

Abstract

Sand at Mendenhall Lake is reported to be glacial moraine (Miller, 1957), bedrock ground-up by the glacier and dropped on the shore before the glacier melted back in the 1940’s. This “ground up rock” idea is a theory: there is no way to confirm it by direct observation.

Direct observation does not support the theory.

Photos of Mendenhall Lake sand and gravel document the formation of sand grains composed of silicates and magnetite in moss and peat along the shore. Radiocarbon dating indicates sand on the surface of the beach likely formed since 1950. The grains are all connected together and have molded, not fractured, surfaces. Adjacent grains conform to each other except where spaces between grains are filled with a dark silicate matrix. Organic fibers and the silicified remains of organic fibers also connect adjacent grains. Silica and magnetite sand appears to be forming at all levels in “lanky moss” and the peaty sand beneath it. Lanky moss at Eagle River Beach, about 40 kilometers away on the coast shows similar sand formation. It appears therefore that the glacial location is not as important as the community of organisms that live in the moss.

References

Barnwell, William W., and Boning, Charles W., 1968, Water resources and surficial geology of the Mendenhall Valley, Alaska: United States Geological Survey.

Connor, C. and Daniel O'Haire, 1988. The Roadside Geology of Alaska. Mountain Press Publishing Company, Missoula, Montana.

Drinkwater, J.L, Brew, AB and Ford, A.B., 1995. Geology, Petrology and Geochemistry of Granitic Rocks from the Coast Mountains Complex near Juneau, Southeastern Alaska. USGS Open File Report 95-638. 126 pages

Drum, R. W., 1968. Silicification of Betula wood in vitro. Science, Vol171 No. 3837 (July 12).

Gehrels, G. and H. Berg, 1992. Geologic Map of Southeast Alaska., U.S. Geological Survey, Denver, Colorado.
 
Krauskopf, K.B., 1959. The Geochemistry of Silica in Sediments. SEMP Silica in Sediments (SP7).

Miller, R.D. 1973. Gastineau channel formation, a composite glaciomarine deposit near Juneau, Alaska. USGS contributions to stratigraphy, U.S. Govt. Printing Office.

Miller, R.D. 1972. Surficial Geology of the Juneau Urban Area and Vicinity, Alaska. Open- file report U. S. Geological Survey (map and booklet).

Motyka, R.J., O'Neel, S., Connor, C.L., and Echelmeyer., 2002. 20th Century Thinning of Mendenhall Glacier, Alaska, and its Relationship to Climate, Lake–Calving, and Glacier Run–Off. Journal of Global and Planetary Change. V. 35, pp. 93–112.
 
USDA Forest Service Region 10, 2013. Mushrooms of the National Forests in Alaska, PDF.