Humberside Geologist no 16

The Hornsea Tesco Boulder.

By Dennis Haughey, Mike Horne and Stuart Jones.

In 2009, the Hull Geological Society was contacted about a boulder in Hornsea beside the road junction on Southgate and Hull Road (NGR TA20244704). It was thought by locals that the future of the stone was threatened by the development of the nearby site by Tesco. This rock was known to locals as a boundary marker and, according to local legend, had stood in the middle of the junction until it was moved to the side of the road when it was metalled about 100 years ago.

It is a rounded brown erratic about 1.35m x 1.15m and standing about 0.6m high above ground level. There are definite scratches at the back of it - perhaps from transportation in a glacier. When we first visited, it was not in very salubrious surroundings - there was a lot of broken glass around it and it had a distinctive aroma! In 1896 W H Crofts recorded “at south end of the old village, at junction of the Hull Road with the main street, a boulder of dolerite 57” by 42” by 18".” This seems to be the same boulder.

ice scrtaches on the boulder

Photo by S Jones

When asked to identify the rock we explained that geologists would need to see a fresh un-weathered sample, but it would be a shame to damage it by hammering. Felicity (Fliss) Clayton (Renaissance Coordinator at East Riding of Yorkshire Council) agreed to members of the HGS removing a core from the boulder. Stuart Jones and Dennis Haughey met Fliss at the site, which had now been cleaned up and planted with flowers. They dug down a little and then drilled out a 2" core from the underside of the boulder. Then they knocked off the outer weathered surface of the core and cemented it back in place, using the powder from the drilling to mix the cement.

collecting the sample

Dennis took half of the core and made a thin section and a polished disc from the sample. We then looked at these at one of the HGS Friday Microscopy evenings. Here are our conclusions:

Hand specimen - overall grey colour. On a broken surface we could see white minerals and pink patches, as well as some well defined, black crystals with a vitreous lustre. The pink patches are not a pink mineral, but a pink staining. The rock weathers brown: onion skin weathering.

Polished section - overall dark grey colour. Grain size generally 0.5 mm but with some long thin crystals up to 2 mm long. It contains cubic crystals of pyrite, these make up about 1% or 2% of the rock.

Thin section - the dominant mineral is rotting elongated feldspar, some crystals of which exhibit multiple twinning, but due to the weathering to clay most have the appearance of simple twinning. Opaque minerals make up 10% to 15% of the rock, some of these are square in cross section (probably pyrite). There are some zoned minerals with second order birefringence. A greenish mineral with a "crazy paving" appearance is also present (probably olivine), as are small amounts of strained quartz and biotite (brown pleochroic with good cleavage).

We later compared it with specimens in the University Collection and some samples of the Whin Sill collected by Dennis. Our conclusion was that we could not prove the erratic was not from the Whin Sill.

Tesco agreed to reposition the boulder and a local historian wrote the (geologically inaccurate) information displayed on a stainless steel structure which was unveiled on 24th February 2012 (HARP 2012).

the Hornsea Tesco Erratic

Picture of Stuart at the site in 2019


References –

Anon 1896 – Report of the East Riding Boulder Committee 1896. Transactions of the Hull Geological Society, 3, 6-9

HARP 2011 - Glacial erratic p 6-7 of Hornsea Area Renaissance Partnership 2011/12 report and 2012-13 Action Plan, 12pp

HARP 2012 (press release) – Hornsea unveils its Glacier Erratic

If you enjoyed reading the article please consider joining the Society or sending a donation.

 copyright Hull Geological Society 2020

Humberside Geologist Online