Identifying our Erratic Boulders

Roland Rockhound's Boulder Hunt

On this page we show you what our Welsh boulders look like. There are some important clues that tell about their origin as volcanic ash 450 million years ago. Also their shape tells us about how they were moved by ice, say 450 thousand years ago.

The Welsh volcanic stones from Arenig have been carried some 100 km (over 60 miles) from their source! And yet boulders up to three metres across like this one (which is on private land) have survived.

A boulder from North Wales three metres across

In order for them to have survived they need three special properties:

  1. The rock needs to be strong without weak zones. Most rocks have cracks called joints. These are widely spaced in the Arenig volcanic rocks.
  2. The rock needs to be hard. The volcanic rocks are silicified, that is impregnated with silica – the same material as quartz or flint. This is harder than steel!
  3. The rock must be insoluble. Silica only dissolves a little in water.

The Arenig rocks are tuffs, which means hardened volcanic ash made of particles (clasts) of all sizes flung out of a volcano. The ash in any one boulder built up over a period of hours to days and in some cases like the photo above you can see the layers of the deposits (which now are neraly vertical). Layers with larger fragments alternate with finer layers.

The larger pieces are pieces of pre-existing rock known as lithic clasts so the deposits are lithic tuffs. These clasts are not as hard as the fine silica-rich matrix and so often weather out as holes on the surface as is seen in this fine example below from Woodgate Valley Country Park.

Volcanic erratic boulder with holes where rock fragments thrown out of the volcano have weathered out.

Often the surface is more uniform with small pits where rock fragments have weathered out as in this example from near Bournville station. The scale shows centimetres and inches.

Most of the surface is uniform because of the growth of fine crystals of quartz, probably from hot waters soon after the volcanic eruption. This ensured the hardness of the rock.

A smooth siliceous volcanic boulder with small pits where rock fragements have weathered out.

Up to now we have only discussed the Welsh volcanic boulders, but there are also some boulders that have not travelled so far. The most common are boulders of the dark igneous rock called basalt, which are shown in the 1890 map as being clustered around Rowley Regis and the area to the south and east. The hilltop at Rowley Regis is the site of a body of basalt, previously extensively quarried and used for roadstone. This basalt is unfractured and hard.  It is very dark coloured when a fresh surface is seen as in this example from Romsley Hill.

The photo also shows us how the surface of boulders gets obscured by the growth of moss and lichen. In this project we could only afford to have a few boulders professionally steam-cleaned, but hand-cleaning can be done effectively by volunteers and we are offering training in doing this.

Weathered lichen-covered surface and fresh surface of basalt boulder at Romsley

Now, what about the shape of the boulders? This can tell us about how they were transported by ice in their journey from Wales.

There is a big difference between being carried on the ice and under the ice.

If a boulder falls from a tall cliff, like we see at Arenig Fawr, onto the snowy surface of a glacier below it will have an angular shape reflecting how it broke off. If the boulder stays on the surface of the glacier as it gradually travels from the mountains, it will stay as it is. The boulder is just hitching a ride on the ice. An example of such an angular boulder is shown here from Cotteridge Park.

Angular boulder that had been transported on the ice, now at Cotteridge Park

These angular shapes are created by frost-shattering of the rock as in this modern Arctic  example from Svalbard.

Frost-shattered rock outcrop, Svalbard

Anyone who has walked on a glacier will have been warned about the dangers of falling down a crack or a shaft a to the bottom of the ice. Anything carried to the bottom of the ice will be tortured and transformed. The pressures here, caused by the weight of hundreds of metres of ice, are immense. Layers of ice move one over another and the resulting shearing motion smears out anything soft. Films and channels of water dissolve soluble particles and sometimes precipitate minerals downstream of boulders where the pressure is lower.

The basal zone of glaciers has much rock debris, most of which has been broken off the bed of the ice. In this photo you can see through this basal ice with layers rich in debris (Tsanfleuron glacier, Switzerland)

Translucent ice containing bands of rock debris

The shearing motion within the basal ice breaks the rocks creating large flat surfaces. As the rock fragments grind together, the corners of the larger fragments get rounded off as in the example from Woodgate Valley Country Park.

Boulder with corners rounded by abrasion under the ice

Having got rounded, the boulder can be broken again. This photo (from Cotteridge Park) shows a boulder showing such rounding and a more recent angular fractured surface.

Rebroken formerly rounded boulder, Cotteridge Park

Another distinctive feature of transport under the ice are scratches on the surface of boulders caused by finer debris rubbing against the boulder surface. Such scratches are actually quite unusual in our case, probably because the boulders are so hard, but an example from near Bournville Station is shown below with scratches running from left to right.

Boulder with striations probably created during transport under the ice

Some of our boulders show a white siliceous skin around them. This is not a well-known phenomenon, but one possibility is that silica precipitated during freezing around the boulder. In this example the skin is broken off, revealing a dark interior, where the boulder was damaged during excavation in a Bromsgrove garden.

Erratic boulder from Bromsgrove with white siliceous rind

The formation of a rind would be a similar process to the precipitation of calcium carbonate coatings around limestone cobbles transported at the base of a glacier. This modern example from Switzerland shows white coatings on dark limestone.

White calcareous crust formed by freezing around limestone cobbles