SPRING officially begins this weekend and advance signs are already plentiful - daffodils are in bloom, some of the hedgerows and trees are revealing their new leaves and some may bear blossom, the grass looks greener and the birds are singing.

Indeed, some birds may already be nesting, among the earliest being the blackbird, carrion crow, song thrush and perhaps that quiet and charming little bird, the hedge sparrow.

Once the change into spring gets firmly under way, our plants and wildlife seem to move with surprising speed.

There is an abundance of new growth everywhere, with plants suddenly making an appearance where nothing seemed to be flourishing only days earlier.

We become acutely aware of our wild birds because of their frantic activity in either courtship or nest-building, while all around their songs can be heard loud and clear.

We might notice some young rabbits, already able to fend for themselves, with toads and frogs probably making an appearance in and around our gardens and ponds - some might be spawning to provide an annual reminder of nature lessons in our primary schools!

It is even possible that some early butterflies and bees may be noticed, particularly if there are long spells of bright and warm sunshine, while hibernating creatures like hedgehogs might be tempted to stir from their slumbers.

However, I am writing these notes with patches of snow continuing to linger in my part of Yorkshire. A covering of snow at this time of year is no bad thing, however.

Indeed, some rural experts believe a slow-thawing deep covering of snow is wonderful for the ground because, when it melts, particularly if it does so very slowly, it carries valuable nutrients deep into the soil. Some consider this to be as useful as a coating of manure.

Despite the snow's spectacular, patchy and lingering presence, I know that the arrival of spring is unstoppable and I received a wonderful reminder during this morning's daily walk because a happy thrush was serenading all who cared to listen.

While out and about in the dales and moors during the snow, I realised just how prominent and remarkable are our miles of dry stone walls.

Appearing black against the pure whiteness of the snow when viewed from a distance, they looked like a web of reassuring strength, linking fields and fells, forming well-defined boundaries and even providing shelter for a variety of creatures and plants.

Even though the walls are man-made, they are so much a part of our landscape that they appear to be part of nature's handiwork. This is probably because they are fashioned by hand from materials found in the ground, ranging from the slate of the Lake District to the limestone of the Yorkshire Dales.

They are to be found in some 21 counties in England, with Yorkshire boasting the greatest total length, somewhere in the region of 21,000 metres.

Half of those are within North Yorkshire alone, which is not surprising. After all, North Yorkshire is England's largest county and much of it comprises open moorland and vast swathes of countryside. But other counties with large amounts of dry stone walls include Durham, Cumbria, Cornwall, Derbyshire and Staffordshire, each bearing the distinctive characteristics of their own locality because local stone is used.

It is almost possible to know which county one is passing through by the appearance and characteristics of its dry stone walls.

I am not sure when the first dry stone wall is thought to have been constructed in this country, but I believe examples dating to 600 BC have been identified in Swaledale.

Experts believe the skill was perfected in Neolithic times and there is evidence the Britons were constructing dry stone walls long before the Romans arrived. This means that some walls may be more than 2,000 years old.

Many of our ruined monasteries, some dating from Norman times, have also revealed evidence of dry stone walls within their grounds and in the countryside nearby, so there is good reason for believing that many Yorkshire and Durham examples are more than 1,000 years old.

It makes those which date to the seventeenth, eighteenth and nineteenth centuries look positively modern.

Quite a lot of existing walls were built in the era of enclosure when large open spaces of land were split into smaller units and enclosed, either by hedgerows or dry stone walls.

This dates many of them to between 1780 and 1830, which in turns means that many of the walls we now admire have been standing for a couple of centuries. There could be no finer tribute to those who built them.

What is surprising, or even amazing, is that all these walls were hand-built without the use of cement. Their construction relies entirely on the correct placing of the stones where their own weight and shape secures them, and this is where the skill comes in.

I have seen mile after mile of wall clinging to rock faces, skirting around lofty cliffs, climbing high into mountains and fells and always apparently part of the ground upon which they stand, following its curves, rises and dips.

Many have stiles to permit landowners and others to climb over them, and an equally large number have smout holes - small holes constructed especially to allow passage from field to field by small animals, like rabbits, or larger animals, like sheep, while preventing horses and cattle from straying.

The successful building of a dry stone wall is not for amateurs or DIY fanatics, although the craft can be taught by a skilled tutor.

The strength depends upon a firm base and also on pieces of stone known as throughs. In early times, the base was made by digging down to the firm subsoil and tightly packing the trench with stones, although in some modern instances, the base is now made of concrete.

A through is a larger piece of stone which pierces the entire width of the wall at intervals of about one yard or metre, being placed at every 18 inches (45cm) in height.

These have the effect of binding together the parts below and around them, their weight, shape and length contributing towards this.

In construction, a dry stone wall is virtually two walls in one. The builder starts by making two parallel lines along the proposed site - these are at least 18 inches apart and in some cases considerably wider.

These identify the outer extremities of the wall, which can be as much as 4ft (1.30m) wide in some cases, but it gets narrower as it rises. When finished, the top will be about 1ft (30cm) wide.

During construction, the two outer walls are filled with rubble and smaller stones which act as a binding, while one of the throughs will be included at every 18 inches.

All surfaces facing the sky or weather will be sloping downwards because if water gets into the interior of the walls, it can freeze or cause internal damage.

Sadly, a high percentage of our dry stone walls are in need of maintenance and repair, often being damaged by ramblers climbing over them without using stiles or people stealing the stones for urban gardens.

But a good one will last a long, long time. As one builder said when he'd finished a stretch: "If thoo has onny trouble wiv it in t'next 200 years, let me know."

On the subject of walls, the famous walled garden at Helmsley has just published a fund-raising book of recipes contributed by local people (including me).

The walled garden was built near Helmsley Castle in 1758, but fell into decline until being rescued by Alison Ticehurst in 1994. Now, it is a thriving concern with a small but dedicated staff.

A registered charity, it promotes education and training, employment and horticultural therapy for those with disabilities and special needs.

Although still undergoing restoration, it is open to the public and has 50 varieties of Heritage Yorkshire Apples, the British Clematis Society's national display, some small animals, a plant nursery, a cafe and even a lawn for picnics. It's a lovely place