Chevin Forest Park GeoTrail #17: Rippled Flagstones
The 17th cache of the series, and the last of the 8 EarthCaches, is located at the lower end of a flagged path leading NE off the main trail along the ridge from Surprise View and heading back down into the woods.
Please refer to GCB7RA7 for the background information on the geology of The Chevin, which provides explanations of how and when the various rock formations were formed.
The point of focus and interest here is the footpath of fine flagstones which, on closer observation, are not from around here, leading to some curious questions such as: Why are they here? Where did they come from? How did they get here? What kind of rock is this? What are the strange markings on many of them?
Although I cannot find any info on these particular stones, similar-looking flagstones are found in several locations up on Rombald’s Moor a few km west crossing boggy peat land, and heading up from the Cow and Calf Rocks car park at Ilkley.
So, assuming they are the same, the rock is likely to be Haslingden Blue (Flagstone), a type of Yorkstone which is a distinctive, durable sandstone quarried from the hills around Haslingden in Rossendale, Lancashire (see here for an excellent comprehensive illustrated history of quarrying in that area and here for a short video visualisation of Lee Quarry geology).
The Namurian age Haslingden Flags/Flagstone, part of the Carboniferous Millstone Grit Group, comprises an interbedded variety of lithologies (rock types) from mudstone through siltstone, to ripple cross-laminated and cross-bedded medium-grained sandstones. The flags are divided into the Upper and Lower Haslingden flags, with a wide range of thickness. Their depositional environment (a unique type of elongate delta) is comparable to present-day deltas such as Mississippi and Colorado.
Renowned for its deep blue-grey hue and exceptional hardness, it was widely used during the C19 and early C20, especially in industrial towns across northern England. Its toughness and resistance to weathering made it ideal for paving streets, constructing mills, building retaining walls, and lining canal towpaths.
Its fine-grained texture and aesthetic appeal has also added to its popularity, and use even for decorative applications. It was often laid in setts - square or rectangular blocks - which became iconic features of cobbled roads in northern towns like Manchester, Salford, and Liverpool. These setts, still seen in many places today, have survived decades of heavy traffic due to the stone’s exceptional strength.
Though demand declined with the advent of concrete and asphalt in the mid-C20, it is still highly valued for restoration projects and heritage work. It is a tangible reminder of Britain’s industrial past and the craftsmanship of traditional stone quarrying. As such, it is not just a building material, but a lasting legacy of the region’s geology, economy, and history.
Like in Ilkley, the stone was brought in here, presumably by helicopter, to protect the pathway from erosion and to encourage walkers to stick to the stone path rather than damage the sides – as would happen in muddy conditions. Also, as it is clearly recycled, it may well have been recovered from demolished Lancashire mills.
Yorkstone is a renowned British sandstone quarried primarily in Yorkshire, celebrated for its durability, natural beauty, and historical significance.
Formed over 300 mya during the Carboniferous period, it comprises tightly packed grains of quartz, mica, feldspar, clay, and iron oxides, resulting in a dense and hard-wearing stone.
It can be split along mica-rich layers - it has a slaty cleavage and may therefore be called sandstone slate. Formerly only riven (split with a chisel along the bedding planes between the sedimentary layers - see here) it is now also often sawn.
Historically, it has been used inter alia for paving the streets of London (using setts) to constructing churches, stately homes, and bridges. Its versatility extends to modern uses, including patios, garden paths, walling, and interior flooring. It’s colour ranges from sandy yellows and buffs to greys (eg. Haslingden blue) and browns, influenced by mineral content and quarry location.
It come in different finishes: riven (see above), with a naturally rough texture achieved by splitting along sedimentary layers; and sawn, with a smoother, more contemporary appearance. Reclaimed Yorkstone, salvaged from historical sites, is prized for its weathered character and sustainability, making it a preferred choice for heritage projects.
Its exceptional durability and slip resistance make it good for both pedestrian and vehicular areas. While it may need sealing to prevent staining, especially in sawn varieties, its low maintenance and longevity make it ideal for traditional and modern designs.
Now what about the strange surface marks? These are mainly ripples, with a few possible plant fossils and concretions (nodules) - but also include marks made by stonemasons.
Ripples preserved in sandstone give a captivating glimpse into Earth’s geological past. These marks, etched into sediment around 300 mya ago, are records of the dynamic environments in which they formed - rivers, beaches, deserts, and shallow seas. They not only reveal long-lost landscapes but also the direction of ancient water or wind flow, sediment transport, and depositional conditions.
They are formed through the movement of water or air over loose sediment, such as sand. As the current flows across a bed of sand, it creates undulations on the surface due to variations in flow velocity and turbulence. These become preserved when more sediment is deposited over them and lithification - turning sediment into rock - occurs. Over millions of years, these preserved ripples become visible in the sandstone beds now exposed in cliffs, quarries, and outcrops.
There are two main types of ripple marks: symmetric and asymmetric.
- Symmetric ripples, aka wave ripples, are typically formed in shallow water environments where waves move sediment back and forth. They are evenly shaped, with crests and troughs that mirror each other. Their presence usually indicates a beach or tidal flat origin.
- Asymmetric ripples, aka current ripples, are formed by unidirectional flow, eg. in rivers or wind-blown settings. They have a steep slope on one side (the lee side) and a gentle slope on the other (the stoss side), indicating the direction of flow. These help determine paleocurrent directions.
Ripple marks in sandstone are more than just surface textures - they are invaluable geological tools. By analyzing ripple orientation and morphology, past environmental conditions can be deduced, such as:
- Water depth and flow velocity
- Direction of current or prevailing wind
- Climate conditions (e.g., arid versus marine settings)
- Tidal influences and shoreline migration
In sedimentary rock sequences, ripple marks often help correlate layers across regions and can guide the search for resources like groundwater, hydrocarbons, or mineral deposits.
The UK has several famous outcrops where such ripple marks sandstone can be seen - particularly in areas shaped by ancient seas and river systems, eg. The Yorkshire Dales and Peak District. These regions feature Carboniferous sandstones with well-preserved ripples. In places like Malham Cove or Stanage Edge, ripple marks are visible in cliff faces and boulders, providing evidence of ancient deltaic and fluvial (river) environments. See here for an illustrated and well-explained short piece on sandstone ripples in Upper Swaledale (shown below).
Concretions are hard, compact mineral masses within the porous rock. These nodules or spherical structures typically form around a nucleus, such as a fossil, pebble, shell or grain, as minerals precipitate from groundwater. Common concretion-forming minerals include calcite, iron oxide, and silica (as here).
Concretions are often harder and more resistant than the surrounding sandstone, leading to distinct shapes that may stand out during erosion. They may be formed millions of years after the rock itself and can become huge (see here). Studying them gives insight into the diagenetic history and geochemical environment of sedimentary rocks.
To Log this EarthCache:
Having closely examined the flagstone/s:
a) Describe their appearance (eg. colour, feel, graininess surface marks)
b) Why was/is this type of rock so useful and still popular?
c) What type of ripples do you think are these? Explain briefly the reason for your answer.
d) Find the concretion and briefly describe it. Why is it so distinctive?
e) (Optional) A selfie with the path in the background would always be well received!