Particle Deposition

There are 3 factors which affect the settling rate of a particle Settling rate -- the speed at which a particle will settle to the bottom of a body of water (deposition) There are 3 factors which affect the settling rate of a particle

1) Size – The smaller the particle (clay, silt) the slower it will settle out. Larger sediments (cobbles, boulders) will settle quickly.

As the stream slows down, the larger particles settle first…

2) Shape – Rounder, more spherical particles settle out faster than flat, angular or irregularly shaped particles. Why is this? Friction is decreased when a particle is rounded and therefore it settles faster. It’s like it’s more aerodynamic - like a sports car. But in this case, it’s hydro-dynamic.

Hydrodynamic particles (round) settle faster than flat or irregular shaped particles

Density – If two particles have the same basic size and shape, the denser particle will settle faster. Which will settle faster – a glass marble or a lead marble? (PS – they are the SAME SIZE marble…)

Settling Rate vs. Settling Time There is an inverse (indirect) relationship between settling rate and settling time. Think about it, sediments that settle at a faster rate (rate increases) require less time to settle (time decreases) Time to settle (sec) Settling Rate (cm/sec) Remember what the graph of an inverse relationship looks like…

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Sorting of Deposited Particles

Horizontal Sorting Final deposition of particles (sediments) usually occurs at the mouth of a stream. This is due to the faster flowing stream emptying into a slower larger body of water. Horizontal sorting takes place. The sediments that were once carried down the stream are arranged from largest to smallest.

More Horizontal Sorting

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Vertical Sorting Happens in QUIET WATER (still – lake, pond) Particles sort out from bottom to top in layers On bottom – largest, roundest, densest particles On top – smallest, flattest, least dense particles Can occur when a depositional event, like a landslide, dumps a large volume of unsorted sediments into a still body of water like a lake It creates “graded beds” of sediments

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Vertical Sorting: Graded Bedding One depositional event One depositional event