What was the Western Interior Seaway?
A sea that cut North America in half, from the Arctic to the Gulf, for most of the second half of the age of dinosaurs. You do not have to take that on trust: the rock it left behind is under Denver, in 6 named layers, and you can read the sea arriving and leaving in the order they are stacked.
AI reconstruction — not a photograph · Shutterstock AI/Shutterstock.com · Shutterstock Standard License
The world about 90 million years ago
Map built from data
The world about 90 million years ago — the middle of Denver's deep-water layers. The source calls this map the Mid-Cretaceous.
- Land
- Shallow sea
- Deep ocean
Yellow is land, mid-blue is shallow sea, dark blue is deep ocean. The map is drawn flat, so places near the poles look wider than they really are.
This is one model’s answer, not a photograph. It shows the world about 90 million years ago — the age of the map itself, which is the closest the source gets to the ages on this site.
Map data: Scotese & Wright (2018), PALEOMAP PaleoDEMs — rendered by triassicwater.com · CC BY 4.0
Map data: Scotese, C. R., & Wright, N. M. (2018). PALEOMAP Paleodigital Elevation Models (PaleoDEMS) for the Phanerozoic (Version 2) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.5460860 Licensed CC BY 4.0. Grid 361×181 at 1° — about 111 km per cell at the equator, so nothing the size of a town can be read from it.
On this map 68.7% of the world's surface is below sea level, and you can see North America is not one piece of land.
The sea, layer by layer, under one city
This is Denver’s rock column, oldest at the top. Every name, age and setting below comes from Macrostrat; the words in square brackets are its own description of the kind of place each layer was made in.
Graneros Shale
98.85–93.9 million years ago · [lagoonal/restricted shallow subtidal, shallow subtidal, coastal indet., lagoonal]
Greenhorn Formation
93.9–90.825 million years ago · [offshore indet., offshore]
Carlile Shale
90.825–89.8 million years ago · [offshore]
Fort Hays Limestone Member
89.8–86.725 million years ago · [basinal, offshore indet.]
Smoky Hill Member
86.725–84.125 million years ago · [offshore]
Pierre Shale
84.125–75.05 million years ago · [delta front, foreshore, shoreface]
Read it from the top and it tells a story without anyone having to narrate it. The oldest layer is coastal and lagoonal — the edge of a sea. The middle layers are “offshore” and “basinal” — deep water, a long way from any shore. The youngest is shoreface and delta front — the edge again, coming back.
That covers 23.8 million years, from 98.85 to 75.05 million years ago. A sea arrived where Denver is, got deep, stayed deep for millions of years, and then drained away.
What the geologists call it
“The strata were deposited in the Western Interior Seaway, which during much of the Cretaceous occupied an elongate north–south foreland basin that was bordered on the west by the tectonically active Cordilleran orogenic belt and on the east by the low-lying, stable North American craton.”
U.S. Geological Survey Fact Sheet 2019–3045, doi 10.3133/fs20193045. Credit: U.S. Geological Survey.
Unpacked: a long, narrow sea running north to south, with a young mountain range rising on its western side and flat, old, stable land on its eastern side. The mountains were being pushed up, the land between them and the old continent sagged, and the sea came in.
Real photograph · marekuliasz/Shutterstock.com · Shutterstock Standard License
So which states were under it?
This is where an honest answer is shorter than the one you wanted. No source this site holds contains a list of states, and the map above is drawn at one degree — about 111 km per square — which is far too coarse to trace a state border on.
What this site can tell you is where it has looked. Denver was under it: the six layers above are Denver’s own rock, and the middle ones are open ocean. Kansas has the Dakota Formation, which Macrostrat records as both marine and non-marine — a coast rather than a deep sea.
If you want to know about your own town, the way to find out is the way this site does it everywhere: look at what is under it and what has been dug out of it.
For teachers
For school
- On this map 68.7% of the world's surface is below sea level, and you can see North America is not one piece of land.
- The oldest layer is coastal and lagoonal — the edge of a sea.
- A sea arrived where Denver is, got deep, stayed deep for millions of years, and then drained away.
Classroom prompt
Read Denver's 6 layers from the top. Which show the edge of a sea, which show deep water a long way from any shore, and in what order do they come?
Extension: the world map here is drawn at one degree, far too coarse for a state border. What would you need to look at to decide whether your own town sat under this sea?
Ages: a sea cutting a continent in half works from about seven; reading a rock column as a story suits pupils of about ten upwards. That range is our teaching choice for this page, not a curriculum claim.
Where this comes from
- The rock
- Macrostrat, API version 2.3.9, read . Unit names, ages and environments are Macrostrat’s own. The name of the sea is from a U.S. Geological Survey publication, quoted above and in the public domain.
- How sure is this?
- The world map is a reconstruction — an estimate, not a photograph — and it comes from a different model than the one used elsewhere on this site for individual places. The two disagree often enough that they are never combined: here the rock under Denver is the evidence, and the map is only there to show the shape of the world it sat in.
Where next
Denver’s own page reads the same rock beside the fossils dug out of it. The period this sea belongs to is the Cretaceous, which also laid down the chalk of southern England, and the ancient oceans page lists the other seas this site has written about.