9 June, 1998
Hi to all:
This morning I spent time in the sedimentology lab with Meredith Kelly. She is finishing her masters, she studied some deposits near Upper Lake Victoria in Valley.
She was investigating whether the deposits were glacial moraines or were the result of a different process called lake conveyor deposits.
Glaciers are formed by large amounts of snow becoming compressed into ice. This ice spreads outward, moving slowly, bulge after bulge. Glaciers pick up and scour everything in their path. One thing that is important to remember about this is that glaciers do not have reverse gear. A glacier that is receeding is still being fed with fresh ice from the rear. A receeding glacier melts fast at the front.
This leading edge is the end of the line for all the debris that the glacier has carried along. As the glacier melts at the front the debris is dropped and forms a deposit. The knd of deposit can tell us something about what was happening climatically at that time. For a big end moraine (a distinct ridge) to develop, a complete standoff between advance and melting has to be sustained for a long time. If the debris is scattered about as a a low ground moraine, this indicates quick melting, thus rapid warming.
Lake conveyor deposits were posited by a New Zealand geologist to answer the puzzle of some wierd deposits that are seen in some of the Dry Valleys. What they saw was this: Fields of pyramids ranging in size from ankle high to about knee high made up of some surprising grain sizes. What was surprising was that the pyramids were fine grained below and capped with larger rocks, some of the rocks quite large. How can this be explained? How could the finer grained material keep its shape if standard glacial acitivity deposited it? Anything that could carry the larger rocks would require a high energy environment (fast water melting for instance) that would move the finer grained material out of the way and destroy the little pyramid shape.
What this geologist noticed was that the frozen surface of the Dry Valley lakes are covered with rock debris. As the amount of ice on top of the lake contracts, then that debris falls through the water and is deposited on the bottom of the lake. when the amount of water in the lake changes, these deposits can be exposed to our view.
Meredith is hypothesizing in her thesis that the deposits that she studied were formed by lake conveyor deposits.
Think I'll head out now!!
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