The Aston Webb boulder
This boulder was discovered in 1909 in an excavation near the recently constructed Aston Webb building on the University of Birmingham’s Edgbaston campus. It was not described until the Lottery project of 2022-2023. It is the only glacial erratic on campus and one of the largest in the Birmingham area.
Follow this link to find a rotatable version of this image of a 3D model of the boulder.
This early 20th century photograph shows the boulder resting on glacial deposits with the Aston Webb building in the background
Although some concrete has been used to stabilize the base of the boulder, it is still close to its original position today.
This early photograph shows Professor Charles Lapworth seated next to the boulder. However, no record survives of anything he might have written about it.
After many decades when the boulder was unmarked, it is now a popular attraction, lying on both a walking trail and a cycling trail for erratics, and featured in primary school visits to the Lapworth Museum.
The Puzzle of the boulder
From the start there was confusion about this boulder. A 1909 newspaper report announcing the discovery does not make it clear that the boulder is not one of the usual volcanic ash boulders from North Wales. Written accounts and Fred Martin’s (1890) map show that boulders in the Harborne area were also derived from the Rowley Regis area where basalt intrudes into Coal Measure sandstones and mudstones. The fine-grained black erratic boulder certainly looks like a basalt.
Part of Martin’s 1890 map of erratic boulders
A microscopic thin section of a sample taken at the base of the boulder in 2018 (height of field of view 0.25 mm).
The thin section is clearly a basalt with opaque crystals (iron oxides), coloured olivine crystals and clear feldspar. But retired staff member Dr. Rob Ixer thought more work was needed since a particular mineral (a zeolite called analcite) characteristic of Midlands basalts like Rowley Regis was missing.
More research needed!
In the lottery project of 2021-2023, further research using techniques used at the University, was carried out.
One clue that the boulder was more complex than previously thought is shown in the photograph of scratches (glacial striations) made by ice as the boulder was transported. These striations need oblique afternoon light to become visible. They are well-developed – more so than might be expected for a hard rock like basalt
This thin section (view 5.6 mm across) shows a mudstone which had been cracked whilst still soft and the cracks filled with sand grains.
Two further small samples were taken from different parts of the boulder and thin sections cut. They revealed not basalt, but sedimentary rock types: sandstone and mudstone. Yet, even then experienced geologists could not see the difference from basalt in the boulder! In the field, geologists often rely on weathered surfaces to show up differences between rock types. But the erratic surface is fresh, not having been subjected to soil processes.
A thin section of a sandstone from a different sample, field of view 1.4 mm. The sample was examined under crossed polarized light which causes minerals to have characteristic colours. Different sand grains in a finer muddy matrix can be seen.
XRF analysis of the boulder
The chemical composition of the rock provides an alternative method of identification. Here, a centimetre-wide beam of X-rays is aimed at the rock. Secondary X-rays are stimulated, characteristic of particular elements. The operator was Dr. Ian Stimpson of Keele University
Analysis shows less than 10% of the boulder is basalt!
Taking samples and making thin sections is slow and destructive, whereas it is possible to measure the composition of chemical elements quickly using a portable X-ray spectrometer. We found that the most useful elements for our purpose were silicon and titanium. Because the method is not fully quantitative, we used training samples from basalts in the West Midlands (and the basalt sample taken for thin sectioning) to define the range of basalt compositions. The two sediment samples that had been thin sectioned and 90% of the points analyzed on the boulder had less titanium and more silicon than basalt.
The graphical plot of results shows the field of basalts (red) compared with the points analysed from the boulder. Most data points lie in a different field (blue) which thin sections show represents sandstone and mudstone.
Rowley after all!
The sedimentary rocks of the erratic boulder are exactly like the Coal Measure sandstones and mudstones that surround the saucer-shaped Rowley basalt intrusion. Even better, the Birmingham memoir of the British Geological Survey records baked blackened sediments next to the intrusion in the Hailstone quarry before they were removed by further quarrying. In contrast, the only other possible source of basalt along the flowline of the ice is from Telford, but here the basalt is in limestone.
Why then was no analcite found in the basalt sample from the boulder, unlike the West Midlands basalts? It is probably a matter of sampling – only a small area was sampled. Also, the distribution of analcite may not be universal since it is a secondary mineral. It forms from circulationg hot water after the basalt has formed, rather than the primary magma.
The Hailstone, Rowley Regis
This 18th century etching shows these striking rock pinnacles made largely of basalt at Rowley Regis
The Hailstone quarry
This Google Earth image from 2011 shows the worked-out Hailstone basalt quarry which supplied roadstone for the M5 motorway amongst others
One more puzzle!
The erratic boulders are expected to have been originally encased in a deposit containing gravel, sand and mud (“till”) laid down by ice. Yet, the Aston Webb boulder lies on sand and gravel laid down by glacial meltstreams. Either the meltstream was powerful enough to transport such a huge block or perhaps the overlying till, which is found on the other side of the railway line on the hospital site, has been eroded away, leaving the block behind.
Geological map of campus
This geological map by the British Geological Survey (Crown copyright 2021) leans heavily on data collected during the PhD of Ray Pickering, later a lecturer in Civil Engineering. The letter E marks the position of the erratic boulder.
This is a panorama, from Romsley Hill, south of Birmingham. It shows the Black Country, including Rowley Regis, on the left, and central Birmingham and the University on the right. Ice flowed from left to right and up onto Romsley Hill.