Other frequently asked questions!

How do we know that a boulder is made of volcanic ash?

Newly cleaned boulders at Woodgate Valley Country Park and Balaams Wood, Frankley show features particularly well, but once you get your eye in, they can be seen more widely.

Geologists call ancient volcanic ashes tuff.  There are different kinds of tuff depending on what is the main component, e.g. lithic tuff (rock fragments), crystal tuff etc.

In the photo, F is the dark-coloured fresh rock as opposite to the pale weathered surface.

H  are holes where rock fragments (geologists call them lithic clasts) have eroded out.  These would have been flung out of the volcano.

The arrows point to elongated (flattened) fragments of what was originally pumice, i.e. volcanic glass full of gas expansion cavities.  The eruption was that of a pyroclastic flow  (geologists used to call these nuée ardentes) – a ground-hugging very hot mixture of gas and rock such as that which buried Pompeii.  The pumice was soft and therefore flattened – hence the geologists’ term welded tuff.  This is diagnostic of volcanism!

Welded tuff structure

The only confusion between the flattened pumice fragments of welded tuffs and normal sedimentary deposits might be mud clasts.  These are sometimes found in sandstones, but in that case the matrix will normally be friable. In the case of welded tuffs, the matrix becomes silicified soon after formation and is very compact and hard.

How do we know whether a boulder is an erratic/how can we tell if they have been moved by ice?

 The classic case is that an erratic is made of a rock that differs from the local geology and stands out as “exotic”.  Given that there are so few natural exposures in Birmingham, 21st century lay citizens have no basis to make this judgement.  Local rocks are dominated by red sandstones (in the west) and red clays (in the east) with Coal Measures and basalt in the Black Country, but a significant part of the city is underlain by glacial deposits – “boulder clay” (what geologists call diamictons laid down by ice), grey clays forming in glacial lakes and sands and gravels representing river deposits.

Before the urbanization developments of the last century or so, it was easier to recognize such exotic stones that had “always been there” as it was part of the local knowledge passed down.  This is still the case in some places, e.g. at Illey where farmers’ families have been there for generations. 

We are indebted to the 19th century “stamp collectors”, stimulated by the Erratic Blocks group which received reports annually at the British Association for the Advancement of Science meeting for noting boulders of particular lithology (rock type) and those few individuals, who compiled maps.  These efforts told us that by far the most common rock type in Birmingham and NE Worcestershire are what they called felsic tuffs – siliceous volcanic ashes that could be traced back to the Arenig Fawr massif in Snowdonia.  Less commonly are basalts, and Rowley Regis was cited as a source, and local sandstones, particularly from the Coal Measures.  In smaller particle sizes, e.g. pebbles (4-64 mm), a much wider variety of lithologies are present including quartzites.  There are a few boulders made of quartzite in our area which could well be genuine erratics.

When we applied to the Lottery for funding, we argued for timeliness on the basis that the erratic boulders were not only disappearing but also being diluted by stones with no prior heritage value, placed by developers for aesthetic effect.  This rather random placement of boulders from various locations has confused the picture. Hence without clear documentation, as is attempted in this project, even experts may be confused as to what was present naturally in a particular location.

Positive identification is made more difficult by the wide range of shapes of the erratics – those transported on or within the ice can remain angular, whereas those subjected to abrasion at the base of the glacier have rounded off corners and in rare cases striations. Probably weathering over the last half million years has removed delicate striations in many cases.

Why is it OK to move boulders?

 The most common reason to be concerned about leaving something intact is if it has value for archaeology. As far as we know, none of the boulders have archaeological significance, i.e. there is no sign that any of them were moved to create new structures or associated with rituals.

Another reason might be that it upsets the pattern of distribution that was created by the ice. However, this is a very large-scale feature as you can see from the maps.  We have moved boulders up to a few hundred metres in Woodgate Valley Country Park, but this does not affect their geographic value and in any case the movement has been recorded in the project.

For example, a boulder (number 75 in our database) at the riding stables at Hole Farm was practically invisible behind a fence and covered by vegetation. Historically it may have been used as a boundary stone, close to the Bournbrook, but it had become inaccessible to the public.

It has been moved to a prominent cross-roads still adjacent to the Bournbrook.  The stone had evidently been painted more than once. Most of this was removed by pressurized boiling water cleaning which reveals the differences in natural weathering colour.

Cleaning boulder that had been painted, Woodgate Valley Country Park
cleaned boulder 75

What influences where the boulders are found?

 The large-scale distribution of boulders was shown in the 19th century maps presented above.  Those moved at the base of the glacier (subglacially transported) would have been deposited in the underlying boulder clay (diamicton = poorly sorted sediment , or till = deposit laid down by ice).  Those transported supraglacially (on the ice) or englacially (within the ice) would perhaps have been deposited on top of till.

Direct evidence that some boulders were on the ground surface for a long time can be obtained by a technique called cosmogenic dating.  Cosmic rays are constantly bombarding the surface of the Earth and creating new elements of distinctive mass (isotopes), some of which are radioactive. The abundance of these new isotopes decreases exponentially downward from the surface and can be measured by very sensitive mass spectrometers. Reconnaissance work using this technique has been carried out on samples collected by Sebastian Gibson as part of his PhD at the University of Cambridge with results on three of the largest boulders in the district giving a variety of ages, mostly younger than the 450,000 years that boulders were first brought from Wales.  This is evidence for a second ice age bringing boulders or that boulders were exhumed after an initial period of burial.

We have rather little direct evidence of where boulders were found. There is for example photographic evidence of a discovery in Icknield Street in a historic photo and a rare recent example is a boulder dug from a back garden in Berkeley Close, Bromsgrove.  We also know that boulders were dug up in Cannon Hill Park, on the Bournville site and at Cotteridge Park.

Boulder discovered in a trench in Icknield Street - historic photo
Roland with Berkeley Close boulder

The Aston Webb boulder at the University was found, unusually, within (or on top of) gravels but it would have probably been too large to be transported by streams. It can have been let down by erosion from an upper till horizon (present nearby) that has been eroded away at this particular site.

Historically boulders were moved to clear them from the centre of fields or where they were in the way, but presumably it would have been a disproportionate effort to move them far.  Some smaller boulders have certainly been transported along streamways.

Will there be another ice age?

 We have been living since the Ice Age in a relatively ice-free, warm time called the Holocene epoch. This started about 11,600 years ago, and during this time our fixed civilizations have developed.  Natural climate oscillations (as described above) are caused by slight wobbles in the Earth’s orbit, known as Milankovitch parameters after the Serbian mathematician who calculated their effects.  On that basis, the next ice age should be on its way in the next few thousand years.  However, the disturbance to the Earth’s climate system caused by our burning of fossil fuels has set us on a different course – this is a justification for arguing that the Earth is no longer behaving in a “Holocene” way and we need to define a new “Anthropocene” epoch.  A vote on which archive should be used to define this will be held in late 2022 and it will be ratified or rejected in the next year or two after by the International Commission on Stratigraphy.

Earth system modelling then predicts that there is too much heat in our atmosphere and oceans to allow ice to build up as it would naturally to generate a further glacial period.  The next ice age is cancelled.