CFP GT #2: Geological Background + Tidal Laminites
The 2nd cache in the series and the 1st of the EarthCaches, provides a summary overview of the geology of The Chevin and the processes involved in its formation. This provides the setting for the subsequent EarthCaches in the series. It then focuses on a particular striking and relatively rare geological feature seen at this location.
For background info on the Chevin Forest Park including a map showing parking options, trails and cache locations, see GCB7RA1.
The geological features of The Chevin owe their form and composition to events that began over 300 million years ago when tropical deltas, rising seas, and tectonic forces worked together to form the rocks.
The Earth in Carboniferous times
During the Namurian stage of the Upper Carboniferous period (c. 320 mya), the area that is now northern England (including West Yorkshire) lay in a huge subsiding basin just north of the equator and the region experienced alternating marine and deltaic conditions. Vast river systems carried sediments southward from the eroding Caledonian Mountains (which were of Himalayan dimensions) depositing sand, silt, and mud into the basin below.
The rocks seen along the Chevin’s slopes and at dramatic features like the outcrops at Surprise View belong to the Millstone Grit Series, an up to 1,700m thick sequence (series or succession) of coarse sandstones interbedded with siltstones and shales (mudstones) which were deposited in fast-flowing rivers and delta fronts (aka prodeltas - the part nearest the sea) in repetitive cycles of delta advance and retreat. Over time, the pressure from the weight of overlying sediments compacted/compressed these layers into solid rock. See the stratigraphic figure below, which summarises the Millstone Grit rock sequences across northern England, including Bradford, which is the closest to that found here at The Chevin.
Summary of the Stratigraphy of the Namurian Millstone Grit Group in Northrtn England
(Left column = stages in Carboniferous Periond, next column = regional sub-stages)
After the sediments were laid down, the region was affected by the Variscan Orogeny, a mountain-building event that folded and uplifted the strata during the late Carboniferous to early Permian.
The resulting geological uplift tilted the sandstone beds, creating a resistant ridge - The Chevin – which stands prominently above the surrounding landscape.
Differential erosion between hard gritstone and softer shales led to the distinctive stepped slopes and craggy edges seen across the ridge.
This Carboniferous foundation was much later reshaped by the glacial and periglacial processes of the Quaternary period (last 2.6 million years). Ice sheets advanced through the Wharfe Valley, scouring rock, depositing debris, and enhancing the steep relief.
Glacial meltwater likely carved small valleys and transported erratics - large boulders not related to the gritstone of this area. Deposits of glacial till can be found along the lower slopes.
Last ice age 18,000 years ago
Though having less fossils than the Coal Measures, the Chevin’s gritstones occasionally contain fossils of ferns, horsetails, and marine invertebrates like brachiopods and crinoids. These remnants offer glimpses into a dynamic past - a changing world of tropical forests, river deltas, and warm seas.
At the published coordinates you will be standing in front of a rock face with a very distinctive feature - thin beds of sandstone called tidal laminites. These are fine, rhythmic sedimentary layers formed in environments influenced by regular tidal currents, such as estuaries, tidal flats, and delta fronts. They result from the alternating deposition of sand, silt, and clay during successive tidal cycles - typically a combination of flood (rising tide) and ebb (falling tide) flows. In this case, each laminate was formed in a single tide and is an ‘extremely rare’ sight.
These were deposited in a coastal area of the Carboniferous delta. Tides were probably higher than they are today, so the water would have come in fast over sand flats, as in present-day Morecambe Bay
Each tide deposits a thin layer of sediment, often with subtle differences in grain size, mineral content, or sedimentary structures. Over time, this creates a distinctive pattern of couplets or tidal rhythmites, which may record daily (as here), fortnightly (spring-neap), or even seasonal cycles.
The thicker, coarser layers represent the stronger currents of the flowing and ebbing tide; the thinner, finer layers, the slack water interval at high /low water when the tide is turning. The difference in thickness of the coarser layers was caused by the neap and spring tide cycles
The alternating layers can display cross-lamination (smaller scale cross-bedding - see GCB7RBA), flaser bedding (thin sand layers in a muddy matrix), or lenticular bedding (isolated sand lenses in mud), depending on the strength and variability of tidal currents.
In ancient rock sequences like the Millstone Grit here, tidal laminates provide critical evidence of past depositional environments. Their presence indicates shallow marine or deltaic settings subject to tidal influence - supporting the interpretation of the Chevin’s Carboniferous landscape as a dynamic, low-lying area where rivers met the sea through a huge braided delta system.
To Log this EarthCache:
a) Examine the rock surface of the layers and describe their average thickness, appearance, colour, feel, etc.
b) Count/estimate how many sand/siltstone layers there are and calculate the number of tide cycles this represents.
c) (Optional) As always, a selfie with the rock face in the background would be well received!
Send your answers to me via the GC messaging service or via forshaw.chris@gmail.com - thanks!
