page_title: Petroleum County, Montana
section_title: Geology
hierarchical_section_title: Petroleum County, Montana / Geology
caption_attribution_description: English: A ~11.5cm in diameter fossil Metengonoceras teigenense from the Cretaceous Mowrey Shale, Petroleum County, Montana, USA. Stonerose Interpretive Center, Gary Eichhorn Ammonite Collection #SR EA 07-04-01.
context_page_description: Petroleum County is a county in the U.S. state of Montana. As of the 2010 United States Census, the population was 494, making it the least populous county in Montana and the seventh-least populous in the United States. Its county seat is Winnett. The county's area was partitioned from Fergus County to become the last of Montana's 56 counties to be organized.
context_section_description: A sandstone ridge with steep dips runs along the northern end of the county. This anticline is petroleum-bearing, and divides the county into two areas: to the north, geological formations of Bearpaw Shale or the Hill Creek Formation; to the south, formations containing older Cretaceous rocks. Marine shales of Cretaceous Age are found as outcrops throughout the county. Sandstones are noted in alternate sequences and are identified at deeper depths in Cat Creek.
  A Tyrannosauridae fossil skeleton was discovered in the Judith River Formation, while Alamosaurus was discovered in the Hell Creek Formation.
caption: A ~11.5 cm diameter fossil ammonite Metengonoceras teigenense from the Cretaceous Mowry Shale, Petroleum County
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