Question 4 (10 points) d A student is mapping the deposits of an ancient river.

Question

Question 4 (10 points) d A student is mapping the deposits of an ancient river. How can he tell the difference between the channel deposits and the floodplain deposits? a) b) c) d) The channel deposits are likely to have much more angular grains and the flood-plain deposits to have more rounded grains. The channel deposits are likely to be coarse grained and the floodplain deposits to be very fine grained. The channel deposits are likely to be fine grained and the floodplain deposits to be very coarse grained. The channel deposits are likely to have much more rounded grains and the flood-plain deposits to have more angular grains. Save

Explanation / Answer

Answer 1:

This is because the edges of stones that the river channel has carried will be smoothed out by abrasive forces of the water and other rocks. At the point where the velocity of water has reduced considerably, these rock that are now rounded pebbled will get deposited near the channel. However, in the case of floods, the water can get pretty agitated and have a lot of force which will make the water velocity rise and fall abruptly, causing rocks of any shapes and sizes to be washed as floodplain deposits. The segregation in the case of floods is minimal and hence angular grains can be easily spotted in floodplains.

Answer 2:

However, point “a” can be plausible given the type of sedimentary rock deposit. If the rock fragments are coarser, these rocks have travelled a short distance from their source while finer fragments can be carried away over longer distances. Please check if the first option is “elevation” of “distance”. If it is distance, the first option is just as much plausible.

However, in some cases If the grain size of the sedimentary rock has coarser rock fragments, it can indicate that the velocity of the stream is high which can be due to the high elevation of the source of the river. In this case the exact size of the even the coarser rock is considered. E.g. boulders cannot be moved even by high velocity of water but cobbles can be moved by the same.

Answer 3:

Answer 4:

Please repost the following questions separately.

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