Lumb Beck Layers EC
The published coordinates are for nearby parking. From here, make your way the short distance across the grass to the southern bank of the beck @ or near N 53 58.890 W 2 0.985 from where you can make the necessary observations.
This EarthCache highlights several interesting features seen at or near this shallow bend in the beck, which has its source in several becks flowing down from the moors west of Cowling and, after several name changes, joins the River Aire just over 1km east of Crosshills.
There are at least 4 things of geological interest to note here: a) the variety of pebbles on the inside of the bend b) the sloping moss-covered rocky blocks on the opposite bank c) the multiple layers of rock below the tree growing at the top of the outside bend and d) the cracked sloping rock slabs on the opposite bank a short distance upstream.
The geological landscape of West Yorkshire is notable for the extensive presence of Millstone Grit, a coarse-grained sandstone that dominates upland areas like the Pennines. Its formation is closely tied to processes of fluvial deposition, which took place during the Upper Carboniferous Period, around 320-315 million years ago.
During this time, the area that is now the British Isles lay near the equator and had a warm, tropical climate. Sea levels fluctuated dramatically due to glacial and interglacial cycles (formation and melting of huge glaciers), which caused repeated sea transgressions (sea level rise) and regressions (sea level fall).
When sea levels fell, large river systems flowed from down the ancient highland landmass to the north and west (the 'Pennine High'), depositing vast quantities of coarse sediment, particularly quartz-rich sands and gravels, into a subsiding basin to the south and east.
These were laid down on shifting sandbanks which were later buried by overlying sediments to a depth of up to 5km in places. Subsequent great overburden pressure and stress from the collision of tectonic plates turned it slowly (through compaction and cementation) into hard sandstone.
Further intermittent earth movements caused uplift and the overlying rocks were slowly removed by wind, water and ice erosion until the sandstone was again exposed at the surface as it is today.
Fluvial deposition is the process by which sediment is transported and laid down by rivers. See the excellent explanatory video here. In the case of Millstone Grit, deposition occurred in a large deltaic complex—a vast network of distributary channels, braided floodplains, and swamps that built out into shallow marine environments.
Typical braided river system delta, Waitaki NZ
During times of low sea level, rivers carried eroded material from upland regions down into the basin, where it was deposited in layers. These river systems' high energy (from the large volumes of fast-flowing water) enabled them to carry coarse-grained material, including sand and grit, which settled out when the river's velocity decreased. Over time, these deposits built up thick sequences (series of rock layers) of sandstone beds.
One of the defining characteristics of Millstone Grit is its coarse texture and resistance to erosion. These properties can be traced back to its mode of deposition. The high-energy fluvial environments in which it was deposited caused only the heaviest and most durable particles - such as quartz grains and lithic fragments - to be deposited.
Finer material such as silt and clay was carried further out to sea or deposited in quieter (slow water flow) floodplain or swamp environments, which would later form thinner, darker grey, fine-grained, clay-rich shale (mudstone), and coal seams (from compressed tropical vegetation) inter-bedded with the gritstones.
These alternating layers of sandstone, shale, and coal found in the Millstone Grit Group reflect cyclical changes in environmental conditions, driven by both climatic fluctuations and tectonic activity (the movement and interaction of the Earth's lithospheric [crust and upper mantle] plates).
These cycles, known as cyclothems, show repeated patterns of marine transgression and regression. During regressions, fluvial and deltaic conditions dominated, leading to sandstone deposition. When the sea transgressed, finer sediments accumulated, forming marine shales. Intervening swampy conditions allowed vegetation to accumulate and eventually form coal.
In West Yorkshire, this complex stratigraphy (structure of the rock strata) is clearly visible in the dramatic gritstone escarpments of Ilkley Moor, Stanage Edge, and the South Pennines. These landforms owe their rugged appearance and height to the erosion-resistant nature of the Millstone Grit. Softer shales and mudstones have been worn away, leaving gritstone to form prominent ridges and plateaus.
Stanage Edge, Peak District NP
To Log the Cache, please answer the following:
Look across the beck at the large tree growing on the edge of the opposite bank. You will see below it that the periodic flooding of the beck has eroded the bank and exposed the underlying rock strata.
a) Describe what you can see.
b) Estimate the depth of the thinnest and thickest layers.
c) By comparing their appearance and considering the explanation of fluvial deposition above, do you think that all the layers are composed of the same type of rock? Please explain your reasoning.
d) Looking where this rock face enters the water, then downstream at the moss-covered blocks of rock and upstream at the cracked rocky slab, you will see that all these areas are sloping at approximately the same angle - please estimate this angle.
e) Why do you think the rock is sloping like this?
f) Kindly send a photograph of yourself or your GPS with the beck in the background along with your log.
Please submit your answers to me using the GC messaging service or by email to forshaw.chris@gmail.com - thanks!