Chevin Forest Park GeoTrail #15: Quartz Pebbles
The 15th cache of the series and the 7th of the 8 EarthCaches is located at Surprise View a popular viewpoint at the high point of the Chevin ridge.
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
You are now on the crest of The Chevin formed by Doubler Stones Sandstone. The steep slopes below facing north towards Wharfedale are a scarp face which cuts across the rock beds which dip to the south. Looking to the south you can see a gentle dip slope down towards Guiseley and Yeadon.
What distinguishes The Chevin’s coarse sandstone Millstone Grit is the frequent inclusion of quartz pebbles, embedded within its rough, gritty matrix. These pebbles, often rounded and translucent, vary in size and were likely transported from distant upland sources to the north before settling in The Chevin’s prehistoric river channels.
Quartz is one of the most abundant and widely distributed minerals in the Earth's crust, composed of silicon and oxygen with the chemical formula SiO₂. It forms through both igneous and metamorphic processes, originating primarily from the cooling and solidification of silica-rich magmas. As magma cools, silica crystallizes into quartz, often found in granite, pegmatites, and rhyolites.
Its crystals are typically hexagonal and have a hardness of 7 on the Mohs scale, making them relatively resistant to weathering. This durability contributes to their abundance in sedimentary rocks such as sandstones, where quartz grains often dominate due to their chemical and physical stability.
It is transparent to translucent, with a vitreous luster, and occurs in a wide range of colours due to trace impurities - amethyst (purple), citrine (yellow), and rose quartz (pink) being common varieties.
Because of its resilience, chemical purity, and crystalline structure, quartz plays a key role in geological processes, industrial applications, and as a major component of many rock types. See here for more info on this abundant and fascinating mineral.
Its presence indicates strong water currents capable of dragging and carrying relatively large fragments of such durable material over long distances along the bottom of river channels. As a result, these quartz-rich beds represent episodes of powerful fluvial activity (river flooding events) within the broader depositional environment of the Pennine Basin.
During the Carboniferous era, flood events in what is now northern England were driven by a combination of tectonic, climatic, and vegetational factors. The region lay near the equator and experienced a tropical, humid climate, which led to intense chemical weathering and high rainfall. These conditions promoted the formation of vast river systems with high sediment loads and frequent flooding.
Tectonic uplift associated with the Variscan orogeny - a major mountain-building event caused by the collision of tectonic plates - created significant topographic relief. This uplift increased erosion rates and the gravitational potential energy available to river systems, accelerating and increasing sediment transport. As rivers cut through rapidly uplifting landscapes, they carried large volumes of clastic material (fragments of pre-existing rocks and minerals such as quartz) into low-lying basins.
Additionally, the widespread development of dense, early forests during the Carboniferous contributed organic matter and helped stabilize some soils, but also influenced river patterns through the formation of peat and coal swamps. These swampy environments interacted with fluvial (river) systems, occasionally obstructing channels and causing avulsions (rapid changes of course) or diversions in flow. Overall, this interplay between tectonics, climate, and evolving vegetation created dynamic, sediment-rich fluvial systems across Carboniferous northern England.
The gritstone layers formed from this sediment are visible across The Chevin’s various outcrops and crags including this craggy outcrop of the Doubler Stone Sandstone forming the crest of The Chevin, where weathering and erosion have exposed them. The surfaces often have the telltale sparkle of quartz grains in the sunlight, and in some locations, such as here, abundant quartz pebbles are clearly embedded in the rock, protruding like fossils.
The carving on the GeoTrail marker stone #8 at Surprise View depicts the strong, swirling currents which would have moved quartz pebbles.
At Surprise View, there is also an excellent interpretation panel which highlights the geology and geomorphology of the region and the Quaternary history of Wharfedale below, along which flowed a glacier that once covered The Chevin.
To Log the EarthCache, please complete the following tasks and send your answers to me via the GC website messaging service or by email to forshaw.chris@gmail.com - thanks!
a) Why does this exposure of rock have these distinctive quartz pebbles, compared with others in The Chevin which don’t?
b) In relation to a) mention 3 factors which could have contributed to the events leading to the presence of such pebbles in the rock?
c) Why do the pebbles tend to stand out from the surface of the gritstone?
d) Closely examine the largest pebble you can find near GZ and describe its appearance (shape, feel, colour).
e) Quartz crystals are typically hexagonal, so why are these pebbles so different in appearance?