How to read a cross section
Work from the evidence in the drawing, one rule at a time:
- Superposition. In layers that have not been overturned, the bottom layer is the oldest.
- Original horizontality. Sediment settles in flat layers, so tilted or folded layers were moved later.
- Cross-cutting relationships. A fault or intrusion is younger than the rock it cuts. Baked rock was there first.
- Inclusions. Pieces of one rock inside another are older than the rock around them.
- Unconformities. A wavy line is a buried erosion surface: rock was worn away before the next layer was laid down.
Each section comes from an invented history played forward, so faults really offset the layers and intrusions only cut rock that was already there. The key uses only what the drawing shows. An unconformity's letter stands for the erosion that made it.
Symbols and levels
Rock symbols follow the New York State Earth Science Reference Tables; igneous rock and the ticks for contact metamorphism follow Regents cross sections. Middle school sets (NGSS MS-ESS1-4) have flat layers and one fault or dike. High school sets add tilting, folding, sills, lava flows and all three kinds of unconformity, and practice the evidence-based reasoning about rock ages in HS-ESS1-5.
Questions
How do you put rock layers in order from oldest to youngest?
Start at the bottom: in layers that have not been overturned, each layer is younger than the one under it. Then place each fault and intrusion after the youngest rock it cuts, and each erosion surface after the rock it cuts off and before the layers resting on it.
What do the little tick marks along an intrusion mean?
They mark contact metamorphism: rock baked by the heat of the magma or lava. Baked rock was there before the igneous rock touched it, so it is older. Regents cross sections use the same tick marks.
Why can a layer above an intrusion be older than it?
Magma can squeeze between layers that are already there and harden as a sill. If the layer above the igneous rock is baked too, the magma came in under it later, so that layer is older than the igneous rock.
What are the three types of unconformities?
An angular unconformity has tilted or folded layers under it and flatter layers above. A disconformity has parallel layers on both sides of an erosion surface. A nonconformity has sedimentary layers resting on eroded igneous or metamorphic rock.
What rock forms where magma bakes limestone or sandstone?
By the Earth Science Reference Tables, heat turns limestone into marble and quartz sandstone into quartzite. Hornfels is the general name for rock changed by heat from nearby magma or lava; the June 2025 Regents rating guide accepted it for baked sandstone.
Is a fault older or younger than the layers it cuts?
Younger. The layers had to be there before they could be broken and offset. A layer that lies across the top of a fault without being offset formed after the fault.
Are these official Regents questions?
No. The sections are invented by this page in the style of New York Earth Science Regents cross sections, using the rock symbols from the Reference Tables. The site is not affiliated with the New York State Education Department.
Sources, checked October 24, 2026: NYSED Earth Science Reference Tables, 2001 edition revised 2006 (PDF, rock schemes and map symbols); NYSED Earth Science Regents exams and rating guides (cross sections in January 2025 questions 80 to 82 and June 2025 questions 55 and 56); Steven Earle, Physical Geology, 8.2 Relative Dating Methods (CC BY 4.0); NGSS MS-ESS1-4 and HS-ESS1-5.