How do rock layers tell the story of one place?
1 min 50 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the Earth and Space Sciences Regents. Topic: Sedimentary rocks, with the past questions on it.
What Dr. Mars says (the words of the video)
How do rock layers tell the story of one place?
This hillside has three layers of sedimentary rock.
Dr. Mars reads them from the bottom up.
Law of superposition: the bottom layer formed first.
This is true unless the layers were turned upside down.
Shale is made of clay, the smallest grains.
Clay settles only in nearly still water.
Long ago, this place was a quiet lake.
Above the shale lies sandstone.
Sandstone is made of sand grains, larger than clay.
Later, this place was a sandy beach.
Waves laid sand on top of the lake mud.
On top lies conglomerate.
Conglomerate: rounded pebbles cemented together.
In fast water, the pebbles hit each other and lost their sharp corners.
After that, this place was a fast river.
It laid pebbles on top of the beach sand.
Over time, buried sediments were compacted and cemented into rock.
Much later, erosion cut this hillside.
Same place, three times: lake, then beach, then river.
The energy of the water decides the size of the grains.
Check yourself! Which layer is oldest: A, B, or C?
Look at the order of the layers.
Answer: C! The bottom layer is the oldest.
That is the shale Dr. Mars tapped first.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.