Egg Mountain (Maiasaura Nesting Site)
Teton County, Montana (Two Medicine Formation) · United States
Type
Historic Nesting Site / Research Locality
Age
Late Cretaceous (Two Medicine Formation), ~80–75 mya
Established
Discovered 1978 (Jack Horner)
Best time
Museum of the Rockies: year-round (check hours)
- ◆First evidence of extensive dinosaur parental care
- ◆Maiasaura peeblesorum — the 'good mother lizard'
- ◆Nests, eggs, embryos, hatchlings, and adults at one site
- ◆Two Medicine Formation — distinct from Hell Creek
Photos
Overview
Egg Mountain revolutionized dinosaur paleontology. When Jack Horner and Bob Makela discovered Maiasaura nesting colonies here in 1978, they provided the first strong evidence that dinosaurs cared for their young — nests with hatchlings and juveniles too large to be newborn, implying parents brought food. The name Maiasaura means 'good mother lizard.' The site changed how scientists and the public think about dinosaur behavior.
Geology
Egg Mountain lies in the Two Medicine Formation, a Campanian-age (Late Cretaceous) rock unit distinct from and older than the Hell Creek Formation. Volcanic ash layers from the nearby Elkhorn Volcanics helped date the nests precisely. The nesting horizon is a paleosol — an ancient soil surface where Maiasaura returned seasonally to lay eggs in mound nests.
What You See
Egg Mountain itself is on private land with no public visitor infrastructure. The discoveries are housed in the Museum of the Rockies in Bozeman, Montana, which displays Maiasaura nests, embryos, and growth-series skeletons. Field programs occasionally visit the general area with landowner permission.
Famous Finds
Maiasaura peeblesorum — nests, eggs, embryos, hatchlings, juveniles, and adults. Orodromeus remains were also found nearby. The site established that at least some dinosaurs were social nesters with parental care, mirroring modern birds and crocodilians.
Visiting
Visit the Museum of the Rockies in Bozeman for the specimens and story. The actual Egg Mountain locality is not open to the public — respect private land. Choteau, Montana (south of the site) is the nearest town for context.
Best time to visit
Museum of the Rockies: year-round (check hours). Montana field season June–August if joining an authorized program.
Getting there
Bozeman (Museum of the Rockies): fly to BZN airport. The Egg Mountain locality near Choteau is ~2 hours north via US-89 — viewable only from a distance or on authorized tours.
Map
Teton County, Montana (Two Medicine Formation) — approximate location only. Exact fossil localities are not published to protect sites from looting.
Market & Provenance
Two Medicine Formation material (Maiasaura, Einiosaurus, Achelousaurus) appears on the market from private Montana land. Egg or nest material is extremely rare and scientifically priceless — any offered commercially demands extraordinary documentation.
Collecting & Legality
The Egg Mountain site is private land. Two Medicine Formation fossils elsewhere in Montana may be collected on private land with landowner permission — but nest sites and egg material are scientifically irreplaceable. Federal land rules prohibit commercial vertebrate collecting.
FAQ
Can I visit Egg Mountain?
The site is on private land and not open to the public. See the fossils at the Museum of the Rockies in Bozeman.
Why is it called Egg Mountain?
Horner's team found so many dinosaur eggs and nests on the hill that the name stuck — it was the first major nesting colony discovered.
Is this the same as Hell Creek?
No — the Two Medicine Formation is older (Campanian) and a separate rock unit from Hell Creek (latest Cretaceous).