The story of a mysterious Montana soil pit

Soil scientists love digging soil pits. Leave them alone anywhere with a shovel and they’ll dig a deep hole, just to see what’s down below.

Many never miss a chance to ply their trade.

In truth, digging and analyzing soil pits is one way scientists advance our knowledge of how we interact with Mother Earth. More pits lead to more knowledge, which in turn leads to better management of our basic resource.

Ryan Schroeder is a soil scientist at Colorado State University. He worked as a range technician for the United States Forest Service, Bighorn National Forest in northern Wyoming in the summer of 2017.

Schroeder recalls, “In early August my boss and I rode our horses up to an area known as The Bowl near Bald Ridge. Our main purpose was to collect a camera trap that was monitoring animal populations browsing on the willows in this alpine meadow.

“We also wanted to dig a soil pit to practice our soil description skills. The soils in Bighorn impact the quality of the natural forage grasses that support grazing animals and the water quality of this National Forest. The Bowl is at about 10,000 feet.”

After collecting the camera, they walked up towards Bald Ridge. They found a few gullies and erosional features around the foot slope of the ridge as it came down into The Bowl. The sandy gullies were filled with rocks and rubble.

Rather than dig straight down for their pit, they decided the gully wall made it easy to save time by using their shovels to clean a face on the side of one of the gullies. They were curious why the soils on this slope were experiencing the erosion issues.

“There was green grass on top, with varying shades of soil, from light brown to tan to dark brown. The upper portion of the soil profile was a sandy loam texture throughout. Clay content increased towards the bottom. We marked the horizon boundaries with fir twigs in the centre of the face.

The soil profile of the pit exposes the mystery. Grey lines mark the land’s history. Under the grass is a loamy soil, with a fairly dark brown colour, indicating organic matter content. The soil changes to a sandy loam, and then to a silty loam. The most dramatic finding are pieces of charcoal. Remnants of past civilization? Evidence of cryoturbation? Only further study will tell. The scientists marked each horizon with sticks. | Ryan Schroeder photo

“At the bottom of this soil profile, about one metre below the soil surface, we saw a very distinct change in colour. This dramatic change indicates a change in organic matter content. In addition, the texture changed almost instantly from a sandy loam to a silt loam.

“The most unexpected thing we found in the lower layer were pieces of wood charcoal one metre below the grass surface on top. Remnants of past civilization? Evidence of cryoturbation, or frost churning? Such a change in texture and soil colour normally means that this bottom layer of soil was once at the top and was then buried. This begs the following questions, with no real answers.”

  • What might these abrupt changes and presence of charcoal tell us about the history of this meadow?
  • Was this area once forested and then burned?
  • What happened to bury this charcoal so deep?
  • How long ago might this have occurred?
  • Was this an archaeological site, with a hearth for past residents, that was buried over time? But one metre deep?

“This is a possibility. However, such dramatic changes in soil texture, colour and structure make me lean more towards this being a buried profile. I have seen other buried horizons like it in areas of Utah and Colorado.

Schroeder thinks this profile may have been buried due to some of the same reasons the area is now experiencing erosion. The alpine ridge is a very prominent feature that collects large snow packs during the winter. These can turn into perennial snow or ice fields, given the right conditions. Though not real glaciers, they can grind at the bedrock and make sediments ready to transport down slope when the snow and ice melts.

There may have been a large snow-field along the top of Bald Ridge that melted and transported large quantities of sediment down into the Bowl and started the soil formation process anew. As the snow packs melt completely, water flows down slope and cuts down into the soil — demonstrating the dynamic nature of alpine soils.

“Digging a soil pit can leave us with more questions than answers. Soils can hold information about the history of a landscape, be it the history of humans or the history of disturbance events that an area has experienced. Such features make them important to study and learn from. That’s just one of many reasons that soil scientists dig soil pits and do profile descriptions. The journey associated with the work often is just as thrilling as the work itself.”

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