Letting our Science show: Fabulous South Dakota Beachfront Property


No, this post isn't a scam. South Dakota and other states where the Hell Creek Formation is exposed had some fantastic beaches, you're just about 66 million years too late to enjoy them as beaches on a seacoast. One of the questions we get from people are some variant of "why is Mosasaurus in the game" or "why is there an ocean in your concept art?", and while I've wanted to talk about the Western Interior Seaway for a while now, Ron Blakely's recently released maps of the Seaway during the course of the Cretaceous are the perfect catalyst to start the discussion. We highly recommend looking through the selection of maps available on his site.

As a brief overview, the Western Interior Seaway was a vast, relatively shallow body of water that covered much of central North America during the Cretaceous, with periodic connections to both the Arctic Ocean and the early Gulf of Mexico. About 30 million years before the Hell Creek Formation was deposited, the Seaway fully split North America into two landmasses: Laramidia in the west and Appalachia to the east. The exact extent of the Seaway was always changing, sometimes rapidly in a very short period of time, so its probably not possible to plot its exact position at any particular point in time, other than in rather general terms. What we can say with some certainty is that due to geologic uplift caused by the Sevier and Laramide orogenies, the Seaway began to drain from the interior of North America about 10 million years before the Hell Creek Formation was formed. No more than 5 million years after the K/Pg boundary, the last remnants of the Seaway had fully retreated from the interior of the continent, with only a small embayment occupying what is today the lower Mississippi river valley remaining.

Blakely's maps indicate that the Western Interior Seaway was reduced to a small remnant by the time of the Hell Creek Formation:

Hypothetical extent of the Western Interior Seaway circa 67 million years ago (left) and at the Cretaceous/Paleocene Boundary 66.040 Million years ago (right). Map by Ron Blakely and Colorado Plateau Geosystems.

Based on our research, we think this reconstruction of the Western Interior Seaway is too extreme in depicting the extent of regression. Exactly what the coastline of the Western Interior Seaway at the end of the Cretaceous looked like is difficult to answer because we don't have any known deposits of the Pierre Shale or Fox Hills Formation (the deep water and near-shore deposits of the shrinking Seaway respectively) that contain the K/Pg boundary. Additionally, these two formations are either subsurface or largely eroded away from the central Dakotas south and eastward. What we can safely say is, the Seaway was in close proximity to the Lance and eastern portions of the Hell Creek formation, and we know this for several reasons:

  • Brackish and salt water tolerant mollusks and other easily fossilized invertebrates are common throughout the Hell Creek, Lance, Ferris and Frenchmen formations, (all of which straddle the K/Pg boundary) indicating the nearby presence of a large source of salt water.
  • There are several "tongues" of rock that formed under marine conditions in the eastern part of the Hell Creek Formation, indicating that dramatic, very rapid changes to the shoreline occurred. While the Breien Marine Tongue is better studied, the Cantapeta Marine Tongue in the upper Hell Creek Formation indicates that there were close connections (no more that a few kilometers) to open marine conditions within the last 300 thousand years of the Cretaceous.
  • The K/Pg boundary can be found as much as 3m above the transition from the Hell Creek (well drained floodplain) to Fort Union (swampy coastal lowlands) formations in many parts of the Dakotas, indicating either a long term reduction in sedimentation rate or a sustained sea level rise beginning over 90 thousand years before the end of the Mesozoic.
  • Open marine conditions of the Cannonball Sea are in place within 100 thousand years after the K/Pg in many parts of North and South Dakota that in the Cretaceous were dry land.
  • The Hanna Basin of southern Wyoming preserves trace fossils of marine organisms that bridge the K/Pg boundary, indicating connection to marine conditions extended at least as far north and west as Wyoming.

Whether or not the WIS remained intact from the Arctic to the Mississippi Embayment through the end of the Cretaceous is less clear and requires more study of fossil and geologic data. One of the more interesting theories about its extent is that, due to a rapidly growing delta that deposited material eroding off of the mountains being formed by the Laramide Orogeny, the Seaway actually divided into a northern portion connected to the Arctic Ocean through western Canada and a southern portion that was connected to the Gulf of Mexico. The Dakota Isthmus that formed as a result tenuously connected Laramidia and Appalachia for the first time in almost 30 million years, but was probably not easily crossed by large animals because the resulting land was essentially a giant muddy tide flat cut by deeper channels, embayments, lagoons, estuaries and small islands without a lot of fresh water and only a few salt tolerant species of plants.

Hypothetical map of the Dakota Isthmus at the Cretaceous/Paleocene Boundary 66.040 Million years ago.

So what does this mean for Saurian? Because we have strong evidence of marine conditions existing in close proximity to the Hell Creek Formation during the period of time we are depicting, the shore of the Western Interior Seaway will be a significant feature of the world for the player to explore. We advise caution before heading into the surf, there are things bigger and hungrier than you waiting in the waves.


Boyd, D.W. & Lillegraven, J.A. 2011, Persistence of the Western Interior Seaway: historical background and significance of ichnogenus Rhizocorallium in Paleocene strata south-central Wyoming, Rocky Mountain Geology, vol. 46.1, pp. 43–69.

Erickson, J. M. 1999, The Dakota Isthmus-Closing the Late Cretaceous Western Interior Seaway, Proceedings of the North Dakota Academy of Science, Volume 53, 1999

Hartman, H. & Kirkland, J. 2002, Brackish and marine mollusks of the Hell Creek Formation of North Dakota: evidence for a persisting Cretaceous seaway, Geological Society of America Special Paper 361.

Lund, S.P., et al. 2002, Magnetostratigraphy of interfingering upper Cretaceous-Paleocene marine and continental strata of the Williston Basin, North Dakota and Montana, Geological Society of America Special Paper 361.

Murphy, E.C., et al. 2002, Lithostratigraphy of the Hell Creek Formation in North Dakota, Geological Society of America Special Paper 361.