But here in the more gently rolling hills of mid-Wales, near the Radnor village of Llandinam, there is erosion of a different sort; and in the eyes of the Countryside Council for Wales - the Government's statutory adviser on wildlife and landscape conservation - it is a more serious form of erosion. Long vistas of green bracken and grass, of hillside-hugging oak woods and the seasonal purple haze of heather are being transformed by a veritable sea of wind turbines, 40m (130ft) high.
No fewer than 103 are under construction on the hilltops above Llandinam. They can be seen for miles around. In the headlong dash to generate at least part of our electricity requirements from a clean - and, supposedly, green - technology, 208 turbines at seven Welsh sites have already been approved. Decisions on another 177, at five further sites, are awaited. Others are in the pipeline. To date, only two proposals have been refused.
Most of our electricity is generated by burning coal and from nuclear power. An increasing, but still small, amount comes from burning gas. Oil makes a minor contribution. All four of these sources are non-renewable - that is, they will eventually be used up. The only renewable source that has been much exploited, mainly in Scotland, is hydro-power, where the force of falling water drives turbines.
Of the other renewables - solar and wind, wave and tidal power - only wind energy is beginning to be exploited on a commercial scale. Last year, all renewables together provided 2 per cent of the United Kingdom's electricity. The Science Policy Research Unit at Sussex University says wind turbines can produce electricity at a cost of 3 to 5p a kilowatt-hour (kwh) compared with coal power stations at 3.5 to 4p a kwh. Gas will almost certainly be cheaper, while the true cost of nuclear power generation can be as high as 8p a kwh.
Because wind turbines produce no greenhouse gases and no airborne pollution to cause acid rain, many environmentalists have welcomed their commercial arrival. Jonathon Porritt writes in the Daily Telegraph: 'Wind turbines are springing up in ever larger clusters to convert the power of the wind into clean, efficient energy. Noise pollution is minimal and no one as yet has been killed by a falling blade . . .
'The number of people who view these as carbuncular horrors is at least equalled by the number who see them as objects of beauty.'
The problem, according to the Countryside Council for Wales - which today publishes Energy: Policy and Perspectives for the Welsh Countryside, and Wind Turbine Power Stations: The Countryside Council for Wales' Policy - is that although the UK has the best wind resource in Europe, economic interests have focused on the highest wind-speed areas; and in Wales, these are almost always cherished hill landscapes.
While the council welcomes wind turbines as a source of renewable energy, it believes that the scale of their contribution to meeting energy needs does not justify overturning established planning safeguards. For instance, the 24 wind turbines in use at Mynydd-y-Cemmaes, east of Machynlleth, are spread over an estimated 778 acres of hill land, but will produce 7.2 megawatts. A medium-sized coal-fired power station covering about five acres produces 2,000MW. Nor does coal have to be a big source of acid rain, provided sulphur-scrubbing technology is employed.
But the council's objection to the Mynydd-y-Cemmaes turbines - that they are located just outside the Snowdonia National Park, in a similar landscape - was not sufficient to persuade a public inquiry to stop the development.
The rush for the windy Welsh hills is the result of the Non-Fossil Fuel Obligation (NFFO), conceived as a subsidy for expensive electricity produced by nuclear power, in order to ensure that newly privatised regional electricity companies bought their share of it. The subsidy is available for renewables, too - currently 11p for each wind-generated kwh - but under EC regulations it is to end in 1998.
The council wants the NFFO subsidy for renewables extended to remove the financial pressure on developers to get wind turbines operating quickly, and to exploit the windiest areas for quick profits. It also wants the NFFO to be extended throughout the UK (only recently was it extended to Scotland which, according to the British Wind Energy Association, has the largest wind resource in Europe), and to discriminate in favour of small-scale wind-turbine stations.
The cost of wind energy development depends mostly on the type and size of the turbines and on the location and type of site. The Danish ministry of energy estimates an average pounds 127,000 for each 250kw turbine; but installing them on remote Welsh hills is likely to add to the bill.
The case for renewables has received a fillip from the Renewable Energy Advisory Group. Its report in December says such forms of energy could contribute up to 45 per cent of current UK electricity needs by 2025, though the practical upper limit is more likely to be 20 per cent.
The group admits that siting of turbines can be a problem, but it says that even a 5 per cent energy contribution would avoid greenhouse gas emissions equivalent to 10 million tons of carbon dioxide annually, reduce acid rain and increase regional and national diversity - and therefore security - of supplies.
If wind power is to make a substantial contribution to electricity generation in the UK, vast land areas will be involved and more conflict with cherished landscapes is inevitable. Most experts agree that between 30,000 and 40,000 wind turbines could generate 10 per cent of the UK's energy needs. The bulk would be in Scotland, but Welsh winds could blow perhaps a quarter of them.
Figures from the British Wind Energy Association suggest that 40,000 turbines would occupy about 1,400 square miles, an area the size of Kent. National parks, areas of outstanding natural beauty, heritage coasts and other conservation areas will all be vulnerable. Small-scale developments in lowland farmland or on former industrial land will simply not be enough.
But whether we need to be seeking more energy generation is a moot point. Conservation and efficiency, the Countryside Council for Wales argues, should attract more investment than generating capacity. And the World Energy Congress in Madrid estimated that 95 per cent of energy use in Europe is inefficient, when all losses during generation, transmission and use are considered. As more wind turbines arise on Welsh hilltops, it is worth noting that 21 per cent of houses in Wales lack loft insulation, and 19 per cent do not even have insulated hot water tanks.
If anyone ventures along the path from Bryn-gwyn to Pegwn-
mawr among the sea of shining, towering turbines, they may feel that the money would have been better spent on a bit of lagging.
The author is the chief ecologist of the Countryside Council for Wales.