Sedimentary Rocks

I. Formation of Sedimentary Rocks- Sedimentary = “settling”- Form when solids settle out of a

fluid such as water or air. - The sediments eventually become cemented to form rocks.

• Weathering - is a process in which rocks are physically and chemically broken down by water, air, and living things.

• Erosion - involves weathering and the removal of rock.

• Deposition - when an agent of erosion – water, wind, ice, or gravity – lose energy, it drops the sediments.

• Compaction – is a process that squeezes, or compacts, sediments.

• Cementation – “glue” - takes place when dissolved minerals are deposited in the tiny spaces among sediments.

II. Classification of Sedimentary Rocks

• Can be classified into two main groups according to they way they form.

1. Clastic Sedimentary Rocks – composed of weathered bits of rocks and minerals.• Classified by particle size.• Conglomerate – made up of rounded particles.

• Breccia – made up of angular particles.

Ex: Shale, Sandstone, Siltstone

Conglomerate

Shale with Plant Fossils

2. Chemical Sedimentary Rocks –

Forms when dissolved minerals precipitate from water solutions. (water usually evaporates or boils)

• Ex: Limestone & rock salt.

Fossiliferous Limestone

III. Features of Sedimentary Rocks

• Unique features are clues to how, when, and where they formed.

1. In undisturbed rocks, the oldest layers are found on the bottom and the youngest at the top.

Law of Superposition

2. Ripple Marks – rock formed along the beach or stream bed.

3. Mud Cracks - formed when wet mud or clay dried and shrank, leaving a rock record of a dry environment.

4. Fossils – traces of remains of ancient life.

Trilobite

Where is the oldest layer located on this picture?