Lesson Plan # 18

Aim: Depositions and Sedimentation

Vocabulary: deposition, sediment, precipitate, horizontal sorting, vertical sorting, unsorted, graded bedding, sand bars, sand dunes, outwash

Discussion: Ok so now we have learned about weathering and how the sediments are moved from one place to another. Today we will learn how that sediment settles down and gets deposited.

Motivation: does anyone know why the dead sea is called the dead sea? Why are there huge salt deposits near and in the dead sea[YUN1] ? So in order to understand why there is so much salt in the dead sea we need to review how things get deposited.

1) Deposition: What is deposition? Deposition[mb2] is the depositing of sediment in a place after it has been eroded. This process is also referred to as sedimentation[mb3] . Factors that affect sedimentation are as follows: particle size, particle shape, particle density.

2) Particle size: If a particle in water is very small such as those particles that are soluble or are in suspension rarely if ever get deposited on the water bodies’ floor. On the other hand boulders and cobbles do get deposited. However there is one way that these sediments can get deposited and that is if the water that holds these particles get so saturated and cannot hold those particles in suspension any more then they settle to the bottom of the water. This happens in the Persian Gulf and the dead sea [mb4] where the salt content is very high and if too much evaporation happens then the salt just gets deposited at the bottom of the ocean because of saturation. In this case the salt is said to precipitate to the bottom of the ocean because of precipitation.

3) Particle shape: if the rock in question is round, then it will settle faster than if it is angular since the odd shapes of the rocks produce lots of friction on the rock and slows down sedimentation.

4) Particle density: denser particles settle faster than less dense particles. Thus particles that settle faster require less time to settle (duhh??? but that’s what the book says in fancy words using “inverse relationships)

5) When the velocity of the medium in which the sediments are traveling slow down such as is the case when water enters an ocean from a river, sediments that where carried down stream by a river get deposited there in a fashion dependent on its shape size etc. for example the largest, densest particles get deposited at the mouth of the stream because they are the heaviest and are affected by the velocity the most while the smaller, lighter particles get deposited further downstream. This process is known as horizontal sorting. When material is layered in a top to bottom fashion it’s called vertical sorting. When underwater landslides occur there is a process known as graded bedding that occurs. This means that vertical sorting happens and the lighter sediments are on the top. If another landslide happens the next wave will reflect the same type of sorting as the lower.

6) When rivers have sections that slow down, the areas that slow down have some basic characteristics such as the pilling up of sand in those areas such as sand bars. These rivers, in order to keep boats moving on it need to have their sand bars dredged from time to time. When a river gets its load dumped into an ocean the deposition forms a delta shaped area [mb5] where the sand gets deposited (sort of looks like a triangle)

7) Deposition by wind: when wind blows the sand that gets blown around also shows layering such as what happens in deserts and beaches in the creation of sand dunes. The dunes usually have a gentle slop facing the wind and on the other side a steep slope.

8) Deposition by gravity: no layering

9) Deposition based on glacial movements: not layered. However there is much deposition in other ways though. Remember, Glaciers are slow moving lakes of ice. When these lakes of ice dry up during a warming of the atmosphere, the glaciers leaves many deposits. One of these deposits is called a drumline which are accumulations of soil and rocks which move in front of it. When the Glaciers finish their advance south, the rocks that they schlepped along on their journey remain in their place. Where they stop the rock mounds (called till) form what’s called terminal moraines. These moraines created the geology of the two forks at the tip of long island and stopped up the finger lakes creating what is now known as the great lakes in western NY. A kettle is a low spot in the path of the glaciers where the Glaciers stopped advancing and the if the kettle is low enough to hit the water table it will fill up and is called a kettle lake. Kames or outwash plains are deltas of sediment deposited at the end of a glacier and an esker is a ridge which is made by water underneath the glacier which flowed out while the glacier was on top and created the ridge.

10) when the oceans deposit their sand on their sides what is created is called a beach

[YUN1]Tell about floating in the dead sea.

[mb2]Deposition is mentioned in Chaza”l in Eruvin and in the Chaza”l about Shlomo Hamelech when he married bas Pharoh and the yam was oleh “Sirton” see Rash”I there where mentions sirton is rocks and gravel

[mb3]See OC 160:1 where it discusses sedimentation

[mb4]See the Nevua of Yechezkel 47:8-9 which describes that in Moshiach’s times the putrid waters of the dead sea will have lots of fish and that fishermen from ein gedi will fish along its banks

[mb5]ע’ במ”ב סימן רכא ס”ק הבסוף מה שכתב בכלי”ג ונראה שקאי ע”ז