Church Lane is a narrow, cobbled lane leading from Ledbury High Street to the medieval parish church of St Michael. The lane is full of attractive timber-framed buildings, and among those historic buildings is the Ledbury Heritage Centre, built around 1480 for a prosperous wool merchant.
It is one of the most photographed places in the whole of Herefordshire.
A brief history of cobbles
Cobbles, also called cobblestones, derive their name from the word cob, meaning a rounded lump. The term is further related to the German Kopf, meaning head.
A cobble is a clast of rock defined on the Udden–Wentworth scale as having a particle size of 2.5–10.1 in, larger than a pebble and smaller than a boulder. Colours range from grey to purple to black depending on the origin of the stone.
Cobblestones are made from highly durable stones, usually granite, limestone or basalt. They were usually dug from nearby areas and then cobbled, or roughly shaped into the size needed. Another popular source of cobbles was stones found in river beds or beaches.
The cobblestones were then set in sand or mortar. Sand allows the road to gently give to traffic, preventing the cracking associated with pavement or asphalt. Streets paved with cobblestones have proven their durability and longevity by showing up through worn out sections of paved roads throughout the world.
Common materials used as cobbles
Granite
Granite is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.
Granite is typical of a larger family of granitic rocks, or granitoids, that are composed mostly of coarse-grained quartz and feldspars in varying proportions. These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though a few (known as leucogranites) contain almost no dark minerals.
Granite is nearly always massive (lacking any internal structures), hard, and tough. These properties have made granite a widespread construction stone throughout human history.
Limestone
Limestone is a sedimentary rock consisting of more than 50% calcium carbonate and is typically white, grey, or brown in colour, but it can also be found in various shades of blue, green, pink, or red. There are many different types of limestone formed through a variety of processes.
Limestone can be precipitated from water (non-clastic, chemical or inorganic limestone), secreted by marine organisms such as algae and coral (biochemical limestone), or can form from the shells of dead-sea creatures (bioclastic limestone). Some limestones form from the cementation of sand and / or mud by calcite (clastic limestone), and these often have the appearance of sandstone or mudstone. As calcite is the principle mineral component of limestone, it will fizz in dilute hydrochloric acid.
Basalt
Basalt is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro. The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in the Solar System.
Molten basalt lava has a low viscosity due to its relatively low silica content (between 45% and 52%), resulting in rapidly moving lava flows that can spread over great areas before cooling and solidifying. Flood basalts are thick sequences of many such flows that can cover hundreds of thousands of square kilometres and constitute the most voluminous of all volcanic formations.
River bed stone
When occurring in streams, cobbles are likely to be found in mountain valley streambeds that are moderately steep. Cobbles are also transported by glaciers and deposited as with other grades of sediment as till. If the till is water-laid, finer particles like sand and pebbles may be entirely washed away, leaving a deposit of only boulders and cobbles. The term shingle beach refers to a beach covered with small- to medium-sized cobbles or pebbles (as opposed to fine sand). Glacially transported cobbles tend to share several identifying features including a tabular shape and downward diagonal striations on lateral facets.
Cobble conglomerates may be alluvial in origin or the product of "stone avalanches", a type of debris flow resulting from unconsolidated cobbles and gravel. In such stone avalanches, well-rounded cobbles may travel the farthest on account of their low rolling friction. When the product of alluvial processes, the cobble conglomerate's matrix consists of gravel and coarse sand. In contrast, the matrices of flow-deposited conglomerates are primarily mud.
Your tasks
To claim this Earthcache you will need to examine the cobbles in and around the published coordinates and and answer the following questions:
Q1 - What is the predominant colour of the cobbles? Can you see any other colours? If so, what?
Q2 - Describe the size and shape of the cobbles. Are they uniform in size and shape?
Q3 - Describe the texture of the cobbles. Are they rough or smooth to the touch and why?
Q4 - Based on your observations, what type of stone has been used here?
Q5 - Approximately how wide is the street at this point?
Please either e-mail me with your answers or send me a message using the link above.
Although not a logging requirement it would be nice to see your photographs. However, please do not publish spoiler photos with close-ups of the cobbles.
Congratulations to longwhatton34 and TeresaA for joint FTF.