Salt Creek Oilfield is a classic anticline, or dome, with an oval shape and a crater-like center of impermeable gray Cody Shale surrounded by a horseshoe-shaped escarpment of Shannon Sandstone called the rimrock, which contains marine fossils and is visible from Highway 259. The oilfield lies in the southwest corner of the Powder River Basin, a depression formed about 60 million years ago as compression of the earth’s crust raised the Big Horn Mountains to the west and the Black Hills to the east and also folded the crust within the basin into anticlines and synclines. Erosion of these folds created isolated buttes and knobs, and sandstone features dating to the Cretaceous Period 70-130 million years ago mark the basin’s southwest edge. The same geologic activity that shaped Wyoming’s mountains and basins also concentrated mineral deposits and transformed ancient plants and organisms into petroleum, coal, and natural gas. Geologists use these landscape features to predict minerals, and at Salt Creek the escarpment signals possible oil pools below because oil and gas accumulate beneath the dense, exposed Cody Shale; oil can be found throughout the field’s 20,000 acres. Oil and gas migrate upward through permeable rock from deep underground and collect when trapped beneath impermeable stone, and anticlines are important because their arched shapes form natural hydrocarbon traps. Most Salt Creek oil comes from Cretaceous sea life, especially microscopic algal plants called diatoms that lived 150 million years ago in seawater covering the area; after they died, they settled on the sea floor, were buried by sand and sediment, and were transformed by pressure and heat into kerogen and then into oil and gas. Oil and gas also collect in stratigraphic traps formed where porous sandstone or limestone layers pinch out beneath impermeable rock, and with most Powder River Basin anticlines depleted, regional production now focuses on these traps.