What is a peat hagg, and why it signals active erosion

Walk onto degraded blanket bog and you'll meet them before you meet anything else: standing columns of peat, sometimes a few inches high, sometimes taller than a person, with bare vertical faces and a flat or domed cap of vegetation. That's a hagg. The channel running between two haggs, cut down to bare peat or stone, is a grough. Together they're the signature of a bog that's lost its water table and is now eroding under its own weight.

Haggs don't form on healthy bog. A functioning blanket bog is flat, saturated, and covered edge to edge in sphagnum, cotton grass and sedge. Haggs appear where that surface has already failed once, usually from historic drainage, burning, or heavy grazing that stripped the vegetation and let the peat dry out.

Once dry peat is exposed, wind, frost and runoff start carving it away. The softer material around a resistant core erodes faster, leaving the hard core standing proud. That standing core is the hagg. The gap it stands in is the grough.

Haggs and groughs: what to look for on the ground

A hagg has three tells that distinguish it from a simple bare patch. First, a bare vertical or near-vertical peat face, usually dark brown to black, with visible banding or root material. Second, a vegetated or partially vegetated cap on top, often still holding cotton grass or heather, which tells you this was once continuous bog surface. Third, it stands above the surrounding channel floor rather than sitting flush with it.

Groughs run between haggs and often connect into a network, sometimes following old drainage lines, sometimes cutting across slope wherever water has found a path of least resistance. On steeper ground this network deepens into proper gully erosion, with channels a metre or more below the intact bog surface and headcuts migrating upslope year by year. A grough network that's actively widening or deepening is different from one that's stable and revegetating at the edges. The first is still taking peat and carbon off site. The second is a site that's starting to heal on its own.

Why hagg erosion is a signal, not a static feature

The reason haggs matter for a condition assessment is that they're rarely finished. Peat is weak once exposed. Freeze-thaw cycles crumble the bare face a little each winter. Heavy rain runs off the exposed surface rather than soaking in, so each storm carries a bit more material down the grough and out of the catchment. A hagg face that looked stable two years ago can be a foot further back this year, and the only way to know is to have seen both.

This is what separates a hagg from, say, a patch of bare peat on flat ground with no channel forming. Flat bare peat can sit there for years without moving much once grazing pressure is lifted and a nurse crop of moss takes hold. A hagg and grough system, by contrast, is shaped by water that has somewhere to go, which means the erosion has its own momentum. Left alone, groughs deepen, headcuts cut back into intact bog, and what was a stable peat body starts losing carbon and water storage capacity at the margin, year on year.

What this means for a restoration plan

If a funded scheme includes reprofiling haggs, damming groughs, or revegetating bare peat faces, the condition of that erosion network at the start of the plan is the baseline everything else gets measured against. A reprofiled hagg that's holding its new slope and growing moss is a win. One that's slumped again after a wet winter needs another look before it's signed off. The hard part has always been checking enough of a scheme's haggs and groughs often enough to know which is which, especially across a restoration area that runs to hundreds of hectares of uneven, wet ground.

That's the gap an annual condition read from wide-swath imagery is built to close, flagging where bare peat has spread or a gully has lengthened since the last pass, so a field team knows where to walk before the next monitoring return is due. If tracking peat condition against your plan is part of the job, see how an annual change layer works for a restoration area.