SME
Historical Geology
Stratigraphy
Put the events in a cross-section in order, with superposition, unconformities and cross-cutting relationships.
Put in order
Read a cross-section from the bottom up, then place what cuts across.
1
?
2
?
3
?
4
?
5
?
6
?
7
?
8
?
Assumptions
•
Beds are laid down flat, the oldest at the bottom (superposition, original horizontality).
•
Tilted beds under flat ones were tilted and eroded before the flat ones were laid down; the eroded surface is an angular unconformity.
•
A dike is younger than every bed it cuts (cross-cutting relationships).
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
More problem types
Deposition rate between dated beds
Use this for “An ash bed is 252 Ma and a lava flow 50 m below it is 254.5 Ma. How fast did the beds pile up, and how old is a fossil bed 30 m up?”
Which principle dates it?
Use this for “A dike cuts a sill. Which principle says which is older?”
Name the unconformity
Use this for “Flat sandstone lies on eroded granite. What kind of unconformity is it?”