WE learned that weathering was a process that breaks existing rocks apart

Sedimentary rocks

In class we said that rocks are like history books.“Sedimentary rocks preserve a record of the environments that existed when they formed. By looking at sedimentary rocks of different ages, scientists can figure out how climate and environments have changed through Earth’s history. Fossils of ancient living things are preserved in sedimentary rocks too.” (Windows to the Universe) If we look at the rocks here in the Berkshires, we can tell what the ancient environment used to be.

We discussed that weathering was a process that breaks existing rocks apart. Then we learned that erosion is a process that carries weathered rock (sediment) to new places. When conditions are right, sediment of the same size will collect (deposit). Quiet conditions are suited for small particles like silt and clay. Moderate conditions are suitable for medium sized particles like sand, and faster conditions are right for larger particles (pebbles).

After these particles are deposited, they are often buried. Over thousands or millions of years, the buried sediment eventually turns into rock. Rocks that are made from sediment are called sedimentary rock.

Common sedimentary rocks:

Conglomerate

Sandstone

Shale

Limestone

Coal

Coquina

Rock Salt

Clastic sedimentary rocks are rocks that are made from weathered rock such as pebbles, sand, silt or clay. Below are some examples.

Sandstone is made of sand. It can form in a somewhat high energy environment where sand can stay, but smaller sediment is carried away by wind or water. A possible environment where sands that make sedimentary rocks form would be a beach or desert sand dunes.
Shale is made of mud and clay. It can form in moderately deep ocean water, a swamp, the bottom of a lake or some other muddy environment. They form here because the water is relatively calm and the silt and clay can settle out.
Conglomerate is made of gravel and sand. It may form from sediments at the bottom of a fast stream. The pebbles and sand can stay behind, but all other sediments are carried away.

Non Clastic rocks are formed from living things or chemical means. Below are some examples.

Limestone is often made up of the calcium carbonate of marine organisms. It typically forms in deeper ocean water.
Coquina is made of larger shells of marine organisms. It forms in shallow water where reefs are commonly found.
Coal- is made up of plants and other organisms that have not completely been decomposed. Swamps are where coal begins.
Rock salt- basically table salt. It forms when ocean water evaporates and leaves salt behind.

Environment	Type of Sediment	Rock type produced	Clastic or non clastic
River	Pebbles	conglomerate rock	clastic
Beach, shallow ocean	Sandy	sandstone	clastic
Bottom of the medium deep ocean	Silt/mud	shale	clastic
Bottom of the deep ocean	Crushed white chalk	limestone	Non clastic
Coral reef, shallow water	Shells	Coquina	Non clastic
Swamp	Organic remains or plants and organism still containing some energy	Coal	Non-clastic
Evaporated ocean water	Salt from salt water	Rock Salt	Non-clastic

How to make a sedimentary rock!

Step 1. Weathering. This is basically breaking apart existing rock. It can be mechanical or chemical. Broken pieces of weathered rock is called regolith.

Step 2. Erosion. This is basically transporting the regolith to a new location. This is called erosion. Erosion is caused by running water, waves, wind, glaciers, and gravity. Regolith transported by erosion is called sediment.

Step 3. Deposit the sediment. When running water or wind slows down, it looses energy. The less energy there is, the easier it is for a particle to settle out. For example, water with a lot of energy, like a fast flowing stream, will sweep away most small and medium particles like silt and sand. Only the larger boulders and pebbles will be left behind. A quiet location, like a lake or pond, will allow the small particles to settle out.

Step 4. Burial and compaction. New sediment gets deposited all of the time. The sediment that gets deposited will have new sediment laid on top of it in the future. As more and more sediment piles on top of the old sediment, the sediment on the bottom gets compacted and cemented together to make a rock. This can take thousands to millions of years to happen.

Fossils are found in sedimentary rock. Fossils are the remains of ancient organisms. An index fossil is a fossil that can be used to find the relative age of a rock. Relative dating can only compare the age of one rock to another. You don’t know how old the rock is, only that it is as old, older or younger than another. Suppose that you find some shale in England and California that have the same index fossil in them. You know that both shale layers are about the same age.

To be an index fossil, a fossil must :

Have existed for a short time
Have existed in many places on the planet (wide range)
Be unique
Must be plentiful (a lot of these fossils can be found)

The Law of Superposition is useful in relative dating too. This law simply states that if you have horizontal layers of sedimentary rocks, the ones on the bottom are older than the ones on the top.

The picture above shows horizontal rock layers. The dark brown layers are the oldest ones, and the light brown rocks at the top are the youngest. It makes sense, the bottom ones had to be there first for the other layers to be on top.

A trace fossil is evidence in a rock that a living thing had once been there. Footprints, trails, burrows and even fossilized dino droppings would be examples of trace fossils.

To find the Absolute date, or actual date of a rock, you need to use radioactive dating. Radioactive dating involves the decay of radioactive elements. Some atoms, like uranium and carbon, have isotopes that are unstable and decay into a new type of element. An isotope is an atom that has more or less neutrons than normal. Scientist use the half- life of the radioactive element to determine its age. A half-life is the amount of time it takes for one half of the radioactive elements to decay into the new element. Think about the M&M lab. Each shake of the box results in about half of the M&Ms flipping over. This is a half life. The time it takes to “shake the box” is different for different radioactive atoms. Carbon 14, for example, has a half life of about 5800 years. So if you start with 100 atoms of Carbon 14, you will have 50 atoms of Carbon 14 in 5800 years.

To find the age of a rock or fossil, you need to figure out how many half lives have gone by.

Suppose you found a bone that has twelve Carbon 14 atoms in it. You know that it originally had 100 atoms of Carbon 14. How old is the rock?

Well, after one half-life (5800 years) you would have 50 atoms. After another half-life (another 5800 years) you would go down to 25. The third half life would take you to 12. So three half-lives have gone by, each one taking 5800 years.

So 5800 + 5800 + 5800 = 17,400. The rock is 17,400 years old.