Geology of Furness & Cumbria
This article presents a brief layman's guide to the geology of Cumbria, and describes the limestone and haematite formations found at Lindal and Marton. The Cumbria RIGS Group booklet "Ancient Rocks in South Cumbria" by Jean Slee-Smith was a particularly useful source of local information for this page.
Rocks and Minerals
Minerals are solid regular mixtures of chemicals. They grow from chemical
ingredients called elements, and may be affected by temperature and pressure as
they develop. Individual types of minerals have characteristic features such as
colour, weight, texture, and hardness that uniquely identify them and make them
useful in different ways. Minerals are classified into groups such as sulphates,
silicates, phosphates, carbonates, oxides and hydroxides. Silicates, which
combine silicon with oxygen, make up more that 90% of the rocks of the Earth's
surface. Oxygen can also combine with metals to form oxides such as haematite.
Minerals that are fashioned for use as personal adornment are known as gems.
Different mixtures of minerals can grow or cement together to form rocks. The three main types of rock are classified as igneous, metamorphic, and sedimentary. Granite is an igneous rock. This rock type develops as molten magma and lava cool. Slate and marble are metamorphic rocks. They are formed from existing rocks that are slowly altered by heat and pressure, or sometimes by heat alone. Sandstone and limestone are sedimentary rocks. These are made up of fragments of other rocks, sea shells, ancient creatures, etc, compressed together and cemented by water-borne minerals, so that the soft sediment transforms into hard and brittle layered rocks.
Cumbrian Geology
The types of rocks found in an area depend on the conditions, such as
climate, which existed when they were formed. The rocks and minerals found in
Cumbria are widely varied, reflecting changes that have taken place over
hundreds of millions of years.
The oldest rocks in the Lake District are the Skiddaw slates, in the north and the west. There are hard volcanic rocks in the central area, around Borrowdale for example, which were laid down around 460 million years ago, and are evident today in the craggy mountains. There are further areas of slate covering parts of South Cumbria, including Coniston, Kendal, Cartmel, and Ireleth. Black Combe is a major outcrop of dark grey slate.
Softer rocks were pushed out from the central Lake District during the Ice Age, forming long valleys as the ice flowed southwards, and later allowing the formation of great lakes such as Windermere and Coniston Water. Clay was formed from the finely ground "rock flour" that resulted from glaciation. The last glacial period ended in the Lake District only 10,000 years ago. The relatively young sedimentary rocks found around the rim of the Lake District are mostly sandstone and limestone.
The rocks themselves and the minerals within them were crucial to the development of many Lake District towns and villages. Buildings were often constructed from slate, sandstone and limestone mined locally. Industries developed to exploit the rocks and minerals: copper and slate at Coniston; green slate at Elterwater; lead, zinc and granite at Blencathra; sandstone at St Bees; dark blue slate at Kirkby Moor and Ireleth; white limestone at Stainton; shale (which is mixed with water and fired to make building bricks) at the Furness Brick works at Askam; and iron ore at Lindal, Marton and elsewhere around Furness.
Furness Limestone & Haematite
Mineral deposits from which valuable metals can be extracted profitably are known as ores. Market conditions determine whether it is profitable to extract and process an ore from the ground. Platinum is so valuable that it need only have a concentration of 0.1 parts per million to be exploitable, whereas iron is less valuable and the ore may need to have a concentration approaching 50% to make mining viable.
The principal iron ore type found at Lindal and Marton is known as haematite, because of its blood-red colour (derived from the Greek 'haima' meaning blood). Typically the Furness haematite is found in rounded masses, resembling animal kidneys, and so is known as kidney ore. Haematite deposits in Furness are generally found in areas of limestone, collected into veins, sops, and basins within the limestone or between the limestone layers. These were found to have an iron content of 50-60%, which can be compared with the 30% iron content of iron ores from Cleveland, Northampton, and Lincolnshire. However, the ore in these regions lay in large beds close to the surface and hence was easier and cheaper to extract than Furness haematite.
The bulk of the limestone in the Lindal and Marton area is grey or greenish grey calcilutite, a hard extremely fine-grained limestone. Limestone was often heated to a high temperature in lime kilns to drive off carbon dioxide, which made it suitable for many uses, such as for spreading on fields to reduce soil acidity, and as a flux in iron and steel-making.
There are numerous natural caverns in the limestone, sometimes indicated on the surface of the ground by natural hollows known as sinks or swallows. The early iron ore miners frequently used these caverns to dispose of water from the mineworkings, as water that was pumped into them conveniently disappeared into the bowels of the earth.
The following description of iron ore from the Gillbrow area of Lindal is contained in the book "Geological Fragments of Furness and Cartmel", first published in 1869:
"Red haematite; unctuous; easily scratched by the file; lustre, sub-metallic; colour, purplish red; streak, bright red; fracture, uneven and minutely crystalline; pieces of carbonate of lime and other minerals occur in it, which, getting coloured by the powder, cannot be seen until the specimen is washed."
The same book contains an account of unusual deposits found whilst sinking new shafts at Lindal Cote in 1855, to create a waterway leading down the valley to Urswick Tarn. A clay bed approximately 6 feet thick was found at a depth of 40 feet from the surface, containing pieces of unfossilized wood, associated with numerous leaves, seed-vessels, and other vegetable remains. The layers found are illustrated in the two figures above.
The Lindal and Marton area contained the greatest concentration of vein-like
haematite deposits in Furness. The main ore bodies ran along north-westerly
fault lines through the limestone in Lindal Moor, and a similar southerly group
of faults in the Whitriggs and Crossgates areas. The deposits varied greatly in
size and shape, some close to the surface such as at Poaka, and some that were
several hundred metres below ground level such as at Lowfield. The
veins at Whitriggs were generally narrower and less productive than those of
Lindal Moor, but were of great significance in early mining activities as the
ore was found at a shallow depth and hence was easy to extract.
The photo on the right shows three examples of haematite found in the Lindal & Marton area. The large piece at the top is blue/black haematite, characteristic of Carkettle. The piece on the left is red kidney ore (also shown at the top of this page). The shiny piece on the right is a higher quality kidney ore, containing approximately 70% iron.
Future Prospects
Important geological and geomorphological features are now being identified and preserved throughout the UK. Cumbria RIGS Group is responsible for identifying and conserving such sites in Cumbria. One site of note is about 1 mile from Lindal, on the A595 road cutting at Green Haume. The creation of the cutting in the 1990s exposed an area where old rocks have been folded and eroded and new rocks deposited on top of them. The rocks in this small area differ in age by about 100 million years.
There is little doubt that the majority of iron ore in the Furness area has already been extracted, and it is not economically viable to extract the remainder. However, it is thought possible that limestone formations under the Duddon Estuary could have supported the formation of haematite deposits. Given a suitable economic climate and favourable geological surveys of the estuary, it could be that modern mining techniques will enable the extraction of ore from this previously unexploited area.