This contribution from Andy Gize a Geologist from Manchester University is about the history (for want of a better word) of the building stones used in the construction of the current locks. Whilst the article is based on Hunts Locks, the materials used are the same for all of the locks on the main navigation.
Hunts Lock was built with three types of rocks. The oldest is granite, then limestone, and finally sandstone (see plan below).
GRANITE.
Granite was used to line the top edges of the Ship (Big) Lock. It was suitable because of its toughness, but also perhaps because of its light grey colour. The granite is seen best at the ends of the 'Big Lock' gates, where it has become polished by over a century of pedestrians (Picture 1). Granite is an igneous rock, meaning that it was partially molten as it ascended near the surface of the Earth. The long white crystals are feldspar, which were crystallising in the molten rock, like sugar crystallising at the bottom of a tea cup or in a jar of jam. They became aligned as the granite was moving. Adjacent to the polished granite slabs, small (millimetre) shiny crystals of muscovite sparkle in sunlight. In addition, small glassy crystals of quartz are present in the granite.
Picture 1. Granite paving stone, Hunts Ship Lock. The long white crystals are feldspars. The darker infilling is quartz and muscovite.
Quartz is the hardest mineral in the granite. At both ends of the ship lock are single slabs of a quartz-rich granite, different to the other granites (Picture 2). The higher concentrations of quartz make these two slabs much harder, and less prone to wear by pedestrians.
Picture 2. To the left is the feldspar-rich granite used predominantly in the construction of the Ship Lock at Hunts. To the right is another granite found only at the south upstream lock gates. This second granite contains less feldspar and more quartz, and consequently, is harder wearing.
The size of the feldspar crystals indicates that the rock was quarried from a large granite body. Although granites occur in N. Wales, the feldspar crystals suggest that the granite was shipped from Devon or Cornwall. In SW England, the feldspars are frequently changed to “china clay” (kaolinite). Shipping of “china clay” to supply the Potteries could also have transported the granite stones.
Although the southwest England (Dartmoor, Bodmin Moor and the Scilly Isles) granites are only exposed as isolated bodies, they are all connected at depth as a single large granite body, known as the Cornubian Batholith. The granites were formed during a period of continental collision, which can be considered as a period of mountain building (an Orogeny). The Cornubian Batholith was formed during a period termed the Variscan or Hercynian Orogeny, which occurred approximately 380-350 million years ago in present day British Isles. Other parts of the world showing the effects of this continental collision include the Appalachian mountains in the U.S.A., Nova Scotia and Newfoundland in eastern Canada, some of the mountains of Spain, Portugal, and North Africa.
LIMESTONE.
Limestone was used to line the Barge (small) Lock. Limestone is a relatively soft rock, and is easy to fashion. It was used to make the edges of the locks, and the carved stones along the sides of the lock.
The limestone slabs along the edges contain excellent fossils. At the south end of the lock are two unused limestone slabs. The fossils are best seen on the south side of the top slab, which is upside down. An excellent complete fossil coral is present, made of many branches (termed a colonial coral: Picture 3). Each branch shows the characteristic feature of a coral branch, which is either cylindrical in cross section, or like honeycomb when it grew tightly. In side the coral wall, are blades or laths which radiate inwards. These colonial coral fossils are called Lithostrotian.
Picture 3. Southern side of top unused limestone block, Hunts Barge Lock, showing a Lower Carboniferous colonial coral (Lithostrotian). The picture has been inverted, as the block is upside down.
Below the colonial coral is a band of fossil shells (Picture 4). These shells originally comprised two shell halves, as present day bivalves such as mussels and cockles. The fossil shells contain only one half, suggesting that the fossils are not the remains of a formerly living shell bank, but an accumulation of dead shells. The fossil shells are called Productus.
Picture 4. Southern side of top unused limestone block, Hunts Barge Lock, showing a shell bed (Productus) The picture has been inverted, as the block is upside down.
Looking at the top of the uppermost limestone slab, roughly in the middle, is a cross section of a single larger (3 cm diameter) coral. This is a solitary coral, termed Palaeosmilia.
A third type of fossil is seen best in a slab lining the east wall of the Barge Lock, by the gate and paddle gear (Picture 5). It comprises a complex array of bending tubes, approximately 2-3 centimetres wide. No clear animal or plant remains are seen, unlike the fossil corals or shells. These fossils are termed “trace fossils”. In this case, the tubes are the evidence of organisms burrowing through the fine sediments.
Picture 5. Limestone pavement near the gate and paddle gear, Hunts BargeLock. The curved tubes are a trace fossil created by burrowing organisms.
Today, most corals occur in shallow waters (200m maximum depth) between the Tropics of Capricorn and Cancer. A present day environment, comparable to that in which the limestones slabs were formed, are the Bahamas.
Following the Variscan Orogeny, which formed the granites, what is now the British Isles was covered by a warm Tropical sea. The sea occurred 359-320 million years ago, at a time termed the Lower Carboniferous (or Mississippian). Rocks of this age are widespread in the British Isles, in the Mendips, S. Wales, the Pennines and the Midland Valley of Scotland. They are also widespread in the United Sates, Ireland and Europe. Later (320-299 million years ago), after the formation of the limestones at Hunts Lock, the sea became shallower and large coastal swamps of trees and reeds developed. Comparable environments today include Florida (the Great Dismal Swamp) and Borneo. These coastal swamps occurred in the later Carboniferous, and are now represented by the coalfields of the Appalachians, Britain and Europe.
The limestones used at Hunts Lock were shipped from Anglesey. “My colleague was correct - it was from Wales, from Tracthbychan near Pentreath on Anglesey. A quarry belonging to a Mrs Williams, apparently” – all credit and thanks to Mac!
SANDSTONE.
The newest rock used to build Hunts Lock is a red sandstone. This was used between the limestone blocks in the sides of the locks (Picture 6), and to end the north and south ends of the locks. This is not to be confused with the north end of the island between the locks, and the eastern lock, where recent concrete has been used!
Picture 6. Hunts Barge Lock. The carved stones are soft limestone. The flat rocks between the limestone are local PermoTriassic sandstones.
No limestone was used in the Ship Lock, and the harder granite was not carved. The PermoTriassic sandstone was used, however, below the granite (Picture 7).
Picture 7. Hunts Ship Lock. The top is made from hard grey granite, which was not carved. Underlying it is red PermoTriassic sandstone.
This sandstone is the local Cheshire sandstone, used extensively throughout the county (e.g., the railway arches north of Hunts Lock, Beeston Castle, and the walls and cathedral of Chester). Sandstone quarries occur throughout the county, One possible quarry for the Hunts Lock sandstone may have been at Runcorn, which also supplied sandstone for the harbour in New York (U.S.A.). The Sandstone Trail occurs along a line of hills made from the sandstone.
The sandstone was formed after the Carboniferous, during the Permo-Triassic (299-199 million years ago). At this time, there was one large continent (Pangaea), which lay along the equator. The environment included widespread deserts, from which the sandstones were formed. Rocks comparable to the Cheshire sandstones extend now worldwide, including North America and Eurasia.
It was during the Permo-Triassic that dinosaurs flourished. Although dinosaur bones have not been discovered in Cheshire, their footprints have been recovered throughout Cheshire (see displays in the Manchester and Warrington museums).
The end of the Permo-Triassic is marked by a global extinction, which included the dinosaurs. From this extinction, terrestrial life recovered to be dominated by a new animal, mammals. From these mammals evolved humans.
Summary – Hunts Lock building stones
The rocks used to build Hunts Lock were obtained not only from local quarries in Permo-Triassic sandstones, but Variscan granites were shipped from southwest England, and Lower Carboniferous limestones from northern Anglesey.
The rocks offer insights in to a period of Britain’s past, between 380-199 millions of years ago. The early granites resulted from continental collision to form one large continent on the Earth. The Lower Carboniferous limestones contain a record of life in tropical seas, with abundant shallow water corals. The youngest PermoTriassic sandstones resulted from the deserts which prevailed after the shallowing of the Carboniferous seas. With the end of the Permo-Triassic, came a major life extinction, which included the dinosaurs. In the time since the sandstones of Hunts Lock, mammals evolved, eventually leadings to us humans.
Additional resources:
The British Geological Survey provides information on a wide range of geological issues, ranging from highly specialised, to free downloadable resources for schools and teachers.
For internet resources: http://www.bgs.ac.uk/education.
A geological time scale can be downloaded from: http://www.bgs.ac.uk/education/britstrat/home
An introduction to the Carboniferous and PermoTriassic of the UK, and the formation of Pangaea can be obtained at: http://www.bgs.ac.uk/mendips/AncientEnv/AncientEnv.html
Additional detailed information can be obtained using Wikipedia
