A letter from the Federation of Trail Riders alleged that ‘water is the absolutely main culprit’ for the damage to the unsurfaced roads such as at Tilberthwaite (May 3, 'Water is the main culprit').

While water is, indeed, the major agent in the final removal of damaged soil structure, it is extremely rarely the cause of the original damage. The initial cause of damage is the conjunction of the magnitude of disturbing forces, their area of application and angle to the horizontal of the surface - effects which are easily demonstrated by the science of soil mechanics and by observation in the field.

The residual soil covering of the Lakeland fells survives considerable rainfall without significant detriment. The consolidation of these soils in the past by great depths of ice and the subsequent colonisation of the surfaces by tough grasses, has made the ground resistant to most natural forms of damage.

Destruction of the structure of these soils requires the application of significant additional forces. The wheels of motor vehicles provide such forces, causing damage to depths of several tens of centimetres. The resulting loosening of the soil fabric will then inevitably permit erosion by moving water. Human foot-fall is only marginally enough on most surfaces to cause damage, and the countless passages of voles which run through the undergrowth cause no damage - it’s all a matter of magnitude.

A key point is that the multiple repetition of a small force almost never produces the same result as one application of a large force.

The solution to the problem of damage limitation, if outright banning is not contemplated, is to limit the permitted weight and tyre types for various classes of vehicle to those suitable for the conditions.

Quad bikes and ‘balloon’ tyres on cycles, for example, do very little damage, and in some cases when the moisture content is right, can help restore damage by reconsolidation.

Robert Courtier