A mountain’s steepest slopes may not always be its fastest-eroding ground. In Colorado’s East River watershed, researchers found that weaker shale eroded substantially faster than harder igneous rock, even where the shale formed gentler terrain. Their work suggests that the composition beneath a landscape can be as important as its visible shape.
The finding is also a reminder that mountains participate in the planet’s carbon cycle. Breaking down different rocks can affect carbon dioxide in different ways, although the study does not establish a new global emissions estimate or show that mountain erosion explains modern warming.
The research was described by the University of Massachusetts Amherst on October 30, 2026. Its team developed a method for measuring erosion in fine-grained rocks that have been difficult to study with a commonly used geological clock.
Why a landscape can be misleading
Stand at a mountain overlook and steep terrain seems like the obvious place to look for the fastest erosion. Gravity pulls material downhill, rockfalls can expose fresh surfaces, and a cliff looks visibly active. But a landscape’s appearance is the result of many processes operating over different timescales.
A resistant rock can stand steep because it is strong enough to maintain that shape. A weaker rock may form a rounded slope precisely because it breaks apart and moves more readily. In that case, a gentler profile is not proof of a quiet geological history; it can be an outcome of the material’s tendency to erode.
This distinction is useful far beyond the specific watershed. If scientists use steepness alone as a shortcut for erosion, they may misunderstand which parts of a mixed-rock landscape supply the most sediment. The Colorado study offers a way to test that assumption rather than simply replace it with another visual rule.
What the researchers measured
The East River watershed drains roughly 465 square kilometers of Colorado’s Elk and West Elk mountains. It includes both durable igneous rocks and weaker shale, giving researchers an opportunity to compare contrasting rock types in the same broader setting.
According to the university’s account, shale’s erodibility was about three times that of the harder rocks in this watershed. This is a result tied to the study area and methods; it is not a universal multiplier for every shale, mountain range or climate. Local rock history, weather, vegetation, slope and drainage can all influence how material moves.
The key point is comparative. The result challenges the idea that a steep section must necessarily be wearing away faster than a rounded one nearby. It does not claim that every gentle slope erodes rapidly or that all cliffs are stable.
A geological clock powered by cosmic rays
The measurement relies on beryllium-10, a rare isotope that can form in quartz exposed near Earth’s surface. High-energy particles from space interact with material in rock. Over time, this leaves a measurable signal that researchers can use to investigate how quickly a surface is being removed.
The basic idea is easier to grasp if we imagine an exposed rock as a surface accumulating marks. A slowly eroding surface can preserve a longer exposure history; faster removal changes how much of that history remains. The real calculation requires models of particle production, shielding, transport and exposure, so a single concentration is not a stopwatch that works without interpretation.
Scientists can analyze quartz carried in stream sediment to learn about erosion in the upstream area. That provides a window onto a landscape over much longer periods than a person could observe directly. It also means the sample represents material mixed by natural transport, making the source and composition of that sediment important to interpretation.
The technical obstacle was grain size. Conventional approaches work more readily with the relatively large quartz grains in rocks such as granite and sandstone. The university says the team developed a method to recover the relevant material from fine-grained rocks such as shale, widening the range of landscapes that can be studied.
Why this matters for carbon, with an important limit
Rock weathering is part of Earth’s slow carbon cycle. Chemical reactions involving different minerals can draw carbon dioxide from the atmosphere or contribute to its release. Erosion matters because it exposes and transports material that can then react with water and air.
The researchers argue that if carbon-releasing rocks erode faster than previously accounted for, some estimates of mountain contributions to the carbon cycle may need refinement. That is a hypothesis about how a local measurement could change a larger calculation, not a direct measurement of a worldwide carbon flux.
The chemistry is not interchangeable across rocks. A calculation must identify what minerals are present, how they react, where the products travel and over what period the reaction takes place. Simply multiplying the Colorado erosion result by the area of all mountains would skip those steps.
Geological carbon processes also operate across thousands to millions of years. Their importance for understanding Earth’s long-term climate does not mean this study overturns the evidence that human greenhouse-gas emissions drive current rapid warming. The two questions concern different sources, rates and timescales.
What remains uncertain?
One watershed is a valuable natural laboratory, but other regions may have different geology, rainfall, glacial histories and vegetation. Repeating the method across contrasting mountain systems would help show whether the Colorado relationship is common, exceptional or dependent on particular conditions.
The new analytical technique also needs to be interpreted alongside uncertainties in sampling and the assumptions behind beryllium-10 erosion estimates. Researchers will want to know how representative a stream sample is, whether sediment sources are mixed evenly and how consistently the method works across fine-grained formations.
The carbon implication requires a further bridge between physical erosion and chemical reactions. Even a well-measured rate of rock removal cannot by itself establish how much carbon dioxide enters or leaves the atmosphere. Measurements of composition, weathering and downstream transport are needed to complete that accounting.
A better question to ask when looking at a mountain
Instead of asking only, ‘How steep is it?’, ask what it is made of and how quickly material leaves it. A rugged ridge and a rolling shale slope can be two different responses to erosion rather than a simple ranking from active to inactive.
For researchers, the next test is whether the expanded measurement technique reveals the same contrast elsewhere and whether combining it with chemistry improves carbon-cycle models. For the rest of us, the study offers a useful correction to an everyday intuition: landscapes record change, but their shapes do not always reveal its speed.
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