Rocks of the Day

Along the Silver Bow Creek Greenway

#2 is around 30-35 cm in maximum dimension.

Life in the USA is not normal. It feels pointless and trivial to be talking about small looks at the fascinating natural world when the country is being dismantled. But these posts will continue, as a statement of resistance. I hope you continue to enjoy and learn from them. Stand Up For Science!

These boulders and cobbles are along the Greenway trail about a mile south of Gregson Station east of Fairmont, Montana. They attest to the power of the water that carried them 10 miles (16 km) or more down German Gulch and Silver Bow Creek. There may have been small icefields or glaciers up German Gulch whose meltwater 12,000 to 15,000 years ago helped move these rocks, or it may have simply been much higher stream flows than we see today.

Although ultimately these boulders were brought downstream by natural waters, they’ve been manipulated a lot by humans since then: the placer miners, the men who constructed the railroads (three railroads passed near the spot where this pile is located), and certainly the reclamation and restoration activities in the past 10-15 years have put them in the pile where they sit today – but I don’t think they are very far from where the creek carried them.

#1 is a boulder of the Lowland Creek Volcanics, erupted between 49 and 53 million years ago. This looks like part of a fairly coarse ash deposit.

#2: I think this was originally layered alternating dolomite and sandy dolomite, which sounds like the Cambrian Pilgrim Formation, deposited about 510 million years ago. It seems to be silicified, but the lichens (dark brown) still have preference for some of the layers over others.

#3 is a cordierite hornfels, almost certainly from the Vaughn Member of the Cretaceous Blackleaf Formation, deposited as mud about 99 million years ago and metamorphosed to hornfels by intrusive igneous sills about 75-80 million years ago. This is almost certainly from about 10 miles (16 km) upstream where German Gulch crosses outcrops of the Blackleaf Formation; that’s really the only upstream source for a rock like this.

#4 is pink Cambrian Flathead sandstone laid down 515 million years ago (or possibly the Pennsylvanian Quadrant sandstone, 310 million years old) also from 10-12 miles up German Gulch.

#5 is decomposing granite that could have come down Silver Bow Creek from the Butte area or from similar rocks up German gulch. Either way the granite solidified about 76 million years ago.

#6 is fossiliferous limestone with stringers of chert, certainly the Mississippian Madison Group and probably the Lodgepole Limestone, deposited in a shallow sea when Montana was near the equator, about 330 million years ago.

The mouth of German Gulch is at Durant along the bottom margin of this map. The rocks are from 15 or so km upstream from that point.

Here are two more examples of rocks along the trail, and the story of how they help lead me to conclude that there was a LOT of water carrying them. These are north of Fairmont Road which is near the top of the map above, so they have been carried even further than the rocks discussed above.

The token is about 37 mm across.

One of my ongoing puzzles about the cobbles and boulders along the Greenway Trail in the upper Deer Lodge Valley (between Fairmont Road and Hwy 1 Rest Area near Anaconda, Montana USA) is whether much or even most of the material on the surface is local and was deposited naturally by streams (and/or glacial outwash) or if it was largely brought in by humans as part of the remediation, which has certainly been aggressive and extensive.

The spotted rock at left is likely a hornfels, probably a cordierite hornfels, from the silicified mudstones of the Cretaceous Blackleaf Formation that crop out about 12-13 miles (18-19 km) upstream on German Gulch, analogous to Rock #3 in the top photo. I think it is probably part of the natural alluvial deposit. Even if it was hauled in as part of the material used to rebuild Silver Bow Creek’s flood plain, it probably came from the gravel pits near Crackerville Road (about 2 miles from where the rock sits today), where it still ultimately came from up German Gulch.

The green rocks are coated by copper carbonate/sulfate and are left-over materials from the old contamination, the slickens, in which the old mine-waste copper-rich water deposited on anything and everything. The remediation was an expensive and thorough removal of the contamination, but did not capture every single rock (and a handful of cobbles like this coated with copper salts really is not a significant problem, I think).

What that suggests to me is that the coppery cobbles are “natural,” original alluvial cobbles that got coated with the copper salts historically. In turn, for me that strengthens the idea that most of the other rocks, including the spotted hornfels, are also more or less in the locations where they were deposited by streams naturally and were not hauled in wholesale, in tens of thousands of tons, to cover the land surface as part of the remediation. This matters because the size of the cobbles transported 12 miles tells us things about the volume and strength of the water flow: it must have been much larger than at present, even in flood stages, to carry such large boulders and cobbles that far.

These rocks are about 250 feet (75 m) away from the bike trail, so they are certainly not part of the obviously foreign material brought in to build up the trail bed. The two photos were taken about 5 feet away from each other.

That’s the present working idea anyway, and I’d probably say I have maybe 90% or more confidence in it. Geologists are allowed to (and should) change their minds in the face of new evidence, so stay tuned.