Editing Boulder, Montana (section)

==Geography and geology==
[[Image:Bouldermontana.jpg|thumb|left|Boulder as seen from near Interstate 15]] Boulder lies at an elevation of {{convert|4990|ft|m|0}} above sea level along [[Interstate 15 in Montana|Interstate&nbsp;15]] at its intersection with [[Montana Highway 69]], approximately {{convert|35|mi|km|0}} north of [[Butte, Montana|Butte]], and {{convert|27|mi|km|0}} south of [[Helena, Montana|Helena]]. The town is on the north bank of the [[Boulder River (southwestern Montana)|Boulder River]], a tributary of the [[Jefferson River]]. According to the [[United States Census Bureau]], the city has a total area of {{convert|1.12|sqmi|sqkm|2}}, all land.<ref name="CenPopGazetteer2019">{{cite web|title=2019 U.S. Gazetteer Files|url=https://www2.census.gov/geo/docs/maps-data/data/gazetteer/2019_Gazetteer/2019_gaz_place_30.txt|publisher=United States Census Bureau|access-date=July 26, 2020}}</ref>

In the late [[Cretaceous]] Period (roughly 81 to 74&nbsp;million years ago), molten rock ([[magma]]) rose to the Earth's surface in and near what later became Jefferson County and eventually formed an intrusive body of [[granite|granitic]] rock up to {{convert|10|mi|km|0}} thick and {{convert|100|mi|km|0}} in diameter. This body, known as the [[Boulder Batholith]], extends from Helena to Butte, and is the host rock for the many valuable [[ore]]s mined in the region. As the granite cooled, it cracked, and hot solutions filled the cracks and formed mineral veins bearing gold and other metals. Millions of years later, weathering allowed gold in the veins to wash down to the gravels in the Boulder River valley.<ref>{{cite web|title=Math and Science Initiative: Elkhorn Volcanoes and the Boulder Batholith |publisher=State of Montana, Office of Public Instruction |url=http://mathscience.mt.gov/Elkhorn.html |access-date=December 17, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20071226191413/http://mathscience.mt.gov/Elkhorn.html |archive-date=December 26, 2007 }}</ref> The Boulder mining district was essentially limited to [[placer mining]] of those gravels, because the source [[lode]]s were in other mining districts in the mountains.<ref name=DEQ>{{cite web |title=Historic Narratives, Boulder |publisher=Montana Government, Department of Environmental Quality |date=August 2004 |url=http://www.deq.mt.gov/abandonedmines/linkdocs/75tech.mcpx |access-date=December 17, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20100103015408/http://deq.mt.gov/abandonedmines/linkdocs/75tech.mcpx |archive-date=January 3, 2010 }}</ref>