1. Sequencing Sedimentary Rock Layers

2. Conglomerate

3. Limestone

4. Sandstone

5. Shale
Why don’t people climb on shale?

6. Sedimentary Rocks form layers

7. Sedimentary rocks form from layers of sediment that collect in horizontal layers. Over time, with pressure and chemical cementing, rocks form.
Sometimes, the layers are exposed by erosion, like these canyon walls in Dinosaur National Park.

8. Changes to Rock Layers

9. Igneous Intrusion: injected rock

10. Magma sometimes pushes upwards into cracks in other rocks, and then forms igneous rock. The igneous rock is newer than the surrounding rock and is called an intrusion.
Dikes are angled intrusions, and sills are horizontal intrusions. This picture has both.

11. Contact Metamorphism

12. Contact Metamorphism
Where rock minerals and texture are changed ,mainly by heat, because of contact with magma

13. Uplift: Tilted layers

14. These layers near Denver, Colorado show how horizontal layers can be altered by uplift. Uplift is when an area rises, causing layers to tilt. Uplift often happens around convergent plate boundaries while mountains are built. In this case, the Rocky Mountains rose and changed these layers.

15. Folding: Wavy Layers

16. Sometimes layers of sedimentary rock are twisted
Sometimes layers of sedimentary rock are twisted. Folds are usually formed by horizontal movements, where deformation happens very slowly with heat and pressure, like at a subducting convergent plate boundary
Split Mountain, Dinosaur National Monument

17. Faulting: Broken Layers

18. Enormous stress in the rock from convergent plate boundaries where uplift is causing vertical movement is released suddenly as an earthquake, and the rock fractures. The layers of sedimentary rock no longer line up.

19. And Sometimes there’s all three!

20. Unconformities: Missing Rocks!

21. Unconformity
A surface separating two rock layers of very different ages, indicating that there is a break in the sedimentary geologic record, and that no sediments remain from that period of time.
Missing time period is called a hiatus
Usually caused by rocks being eroded, then deposited on top.
Technically, intrusions are unconformities too.

22. Hutton's angular unconformity at Siccar Point where 345-million-year-old Devonian Old Red Sandstone overlies 425-million-year-old Silurian greywacke

23. An unconformity occurs when erosion removes the top surface of land and the surface is then reburied. This results in large gaps in the geologic record for the area. Can you find the unconformity in this picture?
Its right here!

24. To interpret Earth history, a diagram called a stratigraphy is sometimes used. It uses symbols to stand for rock types and can be “decoded” if you know your Earth history. Take a look at this one and see if you can see the order that events happened. Use this key to identify the rocks:
Conglomerate
Sandstone
Limestone
Shale
The limestone formed first, then sandstone, then shale. Then an earthquake faulted the rocks. The earthquake fault occurred last because the rocks had to all be there before they could be offset.

25. Let’s try a harder one. There are 10 steps, including the events that happened to the rocks. Do you see the unconformity?
Conglomerate
Sandstone
Limestone
Shale
unconformity
They are conglomerate, sandstone, limestone, sandstone, shale, erosion, limestone, sandstone, shale, erosion.

26. Igneous rocks “intrude” into other rocks
Igneous rocks “intrude” into other rocks. Because they are hot, they affect the rocks that are near them. What type of rocks are heated but not melted?
Look at this stratigraphy and see if you can list the steps. The dark lines show contact metamorphism.
Conglomerate
Sandstone
Limestone
Shale
Igneous
Limestone, sandstone, igneous (volcano)

27. So youngest rocks are always on the top, right?
No! Some changes to layers can place older rocks above younger rocks.
Intrusions- intrusion is newer, and may be sandwiched by older rock.
Extreme folding- if core is taken through fold, older rock could be on top, or if part of fold has eroded.
Faults- if core is taken through fault, older rock could be on top