The Geology of Holy Island

Recently, local naturalists and NHSN’s Earth Sciences Section set out to explore the geology of Holy Island in Northumberland. Take a closer look at what they discovered.

The aim of the trip was to see some of the varied geology of the island, such as the cyclic sediments of the Lower Carboniferous (cyclothems) which include fossiliferous marine limestones. There’s also the most northerly of the Whin Sill suite of doleritic dyke echelons (offset), the Holy Island Dyke.

We started the day at St Mary’s parish church (13th century with later additions) next to the famous Priory. The handsome red sandstone ashlar used is reputed to have been brought from mainland shoreline quarries at Cheswick Black Rocks, but could equally have been quarried from the eastern shore line of the island. The purple, reddish and white sandstones were certainly obtained from the Nessend area.

Examining the variation in geology of the parish church

We moved on to the eastern end of Heugh Hill which is an offset segment of the Whinsill dyke that crosses the southern part of the island. A smooth, chilled surface dipping 5° east is exposed at low tide and represents the original upper surface of the dyke. Abundant elongated amygdales and many ‘ropy flow’ structures, all with a southwest trend, could be seen where not seaweed covered.

Studying an offset segment of the Whinsill with another section under the castle in the background

At Heugh Hill, the contact between the dyke and the Carboniferous country rock is particularly well exposed. About 60 m from the western end, there are three small rafts of saccharoidal limestone in the southern contact zone and a little to the east ‘skins’ of limestone indicate that much of the lower part of the exposure is actual dyke wall.

The contact between the Whinsill (top) and the limestone (bottom) The hammer for scale is 30cm long.

The shales to the east of the Acre Limestone are exposed on the foreshore and quite fossiliferous with many small brachiopods, bryozoan, gastropods and occasional orthoconic nautiloids.

Susan’s beautiful bryozoan was just like the picture in the trip notes!
Brenda spotted a larger orthocone that exhibited some internal structure

St. Cuthbert’s island is only accessible at low water. The remains of a hermitage used by St Cuthbert around 685 AD are still visible. No vertical contacts are exposed on the eastern margin of this segment of the dyke echelon, but the dominant joints on the island are sub-horizontal. We saw the flat surface formed of chilled rock with its slight easterly dip and which lies mainly below high water mark. The lower amygdale (holes formed by gas bubbles) zone and more ropy flow structures were seen.

Ropy flow and amygdales at St. Cuthbert’s Isle

Near Jenny Bell’s Well on the mainland and visible from St Cuthbert’s Island, a 5m high cliff in sandstones and shales overlies the Acre Limestone at the back of the shore. Here, Karl showed us a small fault and low amplitude folds in some of the beds. Interestingly, some beds are much brecciated but lie between unaffected beds. There are also ironstone nodules within the shale. The eastern edge of the cliff is cut by more recent channel deposits, of probable glacial origin, down to shore level.

Investigating the brecciated material that lies between unaffected beds

After lunch, we visited the wooden remains of the lime jetty on the eastern side of The Ouse, the bay between the harbour and Castle. Here we searched for exposures of the Sandbanks Limestone with small brachiopods, intruded by the Riding Stone-Cockle Stone segment of the dyke echelon. At the south side of the eastern end of the segment a 4 m wide flat bench of chilled rock was seen to exhibit more ‘ropy flow’ structures.

Standing on the bench of chilled dyke material close to the remains of the jetty before searching for more ropy flow structures

From the high vantage point of Castle Hill on the southeast side of the island, the southward-staggered emplacement of segments of the Holy Island Dyke echelon is clear. At the eastern end of Castle Hill is the large complex of limekilns built in 1860. Large quantities of limestone (for example, 3590 tons for the year ending October 1866), brought from Nessend Quarry via the mineral waggonway (now used as the pathway in the area), were calcined in these kilns. The lime was shipped from the Castle Jetty.

Inspecting the lime kilns at the last locality of the day

While this was the last of this year’s scheduled trips, there are plans to do one or two more over the next few months, so keep an eye on Newsletters and NHSN adverts. We look forward to welcoming you on one soon!

Content ©Karl Egeland-Eriksen, 2021

Photos ©Brenda Turnbull, 2021

Karl Egeland-Eriksen
Earth Sciences Section Coordinator

Born (1961) and educated in Newcastle, I took early retirement from a career in NHS dentistry in 2015 (with a few years working in Australia and New Zealand as well). I became interested in geology whilst studying for an Open University degree in the early 1990s and, on completing that degree, went on to take a Master’s degree in Environmental Sciences at Lancaster University. I have been an active member of the Open University Geological Society since 2006 and have organised around 200 geological field trips in Northern England and the Borders for the local Northumbria branch (and other groups) over the last 10 years.