Hot Springs in Fall River County, South Dakota — The American Midwest (Upper Plains)

Late Mesozoic Era

~100 million years ago

Photographed by William Fischer, Jr., September 25, 2026

1. Late Mesozoic Era Marker

Inscription.

Also known as the age of dinosaurs, the latter portions of the Mesozoic Era mark the presence of shallow seas in the Black Hills area. What will be South Dakota is home to marine reptiles like plesiosaurs and mosasaurs, ammonites, gigantic fish, and even toothed birds! Dinosaurs are found regionally during the Mesozoic, but we only find fossils of marine animals near Hot Springs from the Pierre Shale Formation.

[Illustration caption reads]
Mosasaurus, a marine lizard, and hesperornis, a toothed bird. Not to scale.

Erected by The Mammoth Site of Hot Springs, Inc.

Topics. This historical marker is listed in these topic lists: Animals • Environment • Paleontology • Waterways & Vessels.

Location. 43° 25.469′ N, 103° 29.012′ W. Marker is in Hot Springs, South Dakota, in Fall River County. It can be reached from no nearby street. Marker is along the Mammoth Site entrance walkway. Touch for map. Marker is at or near this postal address: 1800 US Highway 18 Bypass, Hot Springs SD 57747, United States of America. Touch for directions.

Regionally,

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this marker is in West River.

It is also in the American Black Hills,

in the Lewis & Clark Corridor,

on the prairies,

and on the Northern Plains.

Globally, it is in North America,

the Western Hemisphere,

the Western World,

and the Anglosphere.

Historically, it finds itself in what was once Rupert’s Land

and also the Louisiana Purchase.

Other nearby markers. At least 8 other markers are within walking distance of this marker: Paleozoic-Mesozoic Era (here, next to this marker); Mid-Cenozoic Era (a few steps from this marker); Pleistocene Epoch (a few steps from this marker); Paleozoic Era (a few steps from this marker); Precambrian Era (within shouting distance of this marker); Mammoth Site of Hot Springs, SD (about 600 feet away, measured in a direct line); Woodward Field (approx. 0.4 miles away); Black Hills College (approx. 0.4 miles away). Touch for a list and map of all markers in Hot Springs.

Regarding Late Mesozoic Era. Markers and rocks are part of the walk through Black Hills geology leading to the Mammoth Site.

Related markers. Click here for a list of markers that are related to this marker. Follow

Photographed by William Fischer, Jr., September 25, 2026

2. Late Mesozoic Era Marker

the markers chronologically through the geologic timeline

Credits. This page was last revised on October 24, 2026. It was originally submitted on October 21, 2026, by William Fischer, Jr. of Reynoldsburg, Ohio. This page has been viewed 15 times since then. Photos: 1, 2. submitted on October 21, 2026, by William Fischer, Jr. of Reynoldsburg, Ohio.