The mountains surrounding Missoula feature Precambrian-aged, Missoula Group quartzites and argillites. Missoula itself sits upon relatively recent, unconsolidated, basin-filling sediments made up, in part, of sub-rounded gravels derived from those mountains. Pattee Creek, Rattlesnake Creek and the Clark Fork River all drain areas underlain by the quartzites and argillites. Argillite boulders and cobbles of the tributary creeks quickly break down, while the resistant quartzites weather into sub-rounded boulders ranging up to beach-ball in size.
Medium to dark-gray argillites and quartzites crop out along the south bank of the Clark Fork River immediately east of the University, seen first-hand during a stroll along the Kim Williams Trail. Building contractor Daniel Heyfron staked Missoulas first building stone claim (Missoula County records, Quartz Location, Book F, page 215) in 1889 on the portion of this outcrop located closest to downtown Missoula. With his partner, John Matthus, Heyfron quarried the stone and hauled the rough blocks off to Missoula building sites. This stone cleaves readily along the sedimentary bedding planes yielding roughly square blocks suitable for masonry. Most of the stone they quarried went into building foundations.
Most builders found the pink to maroon-colored quartzites more pleasing for building stone. The Bonner Quartzite, a subdivision of the Missoula Group, outcrops along Route 200 immediately east of Bonner and elsewhere around Missoula Valley. An analysis of the quartzite in Missoulas buildings suggests builders never quarried the Bonner Quartzite from outcrops but preferred to collect quartzite boulders from stream and gravel deposits. Aside from Heyfron, no other building contractor ever staked a claim on a Bonner Quartzite building stone deposit.
Bonner Quartzite is extremely hard. Its boulders possess no apparent planes of weakness, making them difficult to shape, even with stonecutting tools. Beginning in the late 1880’s the stonecutters split the boulders to expose the flattened fracture surfaces on the exterior walls. By the early 1900’s stonecutters had mastered the tricky art of splitting and pitching the pink quartzite, while the stone masons had mastered the art of setting the quartzite into beautiful masonry walls. Their fine craftsmanship is preserved in the foundations of hundreds of Missoula residences, churches, schools and commercial buildings.
The quartzite found only a local market. Stone cutters never surfaced, polished or sculpted any of this stone, and they seldom shipped it outside Missoula. This rock, while apparently quite strong (the authors estimate at least 15,000 psi for unweathered and unaltered rock blocks) often is highly jointed and characterized by a tendency to further split along incipient joints. For this reason the quartzites may not have been found usable by local contractors for uses other than the rock rubble masonry foundation footings of major buildings. Quartzite foundations remained popular in residential construction through the early 1920s, but substitute materials, as well as a dwindling number of craftsmen, ended the practice.