From about 81 to about 74 million years ago, magma rose repeatedly through the earth’s crust in the Elkhorn Mountains region, creating the enormous Elkhorn Mountains Volcanics and, later, the Boulder Batholith. Magma that reached the surface caused violent explosions that hurled rock, cinders, ash, and droplets of melted rock into the air, and wind carried great ash clouds eastward, burying many animals; dinosaur fossils in the Two Medicine Formation near Choteau were preserved by these eruptions. The volcanic field was about 100 miles in diameter and up to 3 miles thick. After the volcanic pile thinned, magma no longer reached the surface and instead accumulated near its base, where so much intruded that it formed the granite body now known as the Boulder Batholith, extending from Helena to Butte. This magma was generated as the Farallon Plate moved beneath western North America, with approximately 10,000 miles of the plate moving under Montana. Seismic and gravity studies show that the batholith extends to a depth of about 10 miles and contains tens of thousands of cubic miles of granite. The magma also contained metals, and as the granite cooled, hot solutions entered cooling cracks and formed veins of copper near Butte and gold in this area. Later weathering released gold from the veins into valley gravels, and beginning in 1938 a gold dredge worked through the gravels of Prickly Pear Creek, leaving a churned-up landscape; although dredging ended permanently in the mid-1960s, the machine was not moved to Bolivia by a South American mining company until the early 1970s.