Didcot GeoCache Series
After our recent GeoCache Series at Great Western Park (GWP) (cache #1-11 & 14) and in central Didcot (cache #12-13 & 15), we decided to place a few more caches in other parts of Didcot, in order to honour local heros, and to reflect our rich local history, stories and heritage. We hope you will enjoy these new caches and may also learn something when you are caching, thank you!
Sedimentary rocks
Sedimentary rocks are types of rock formed by the cementation of sediments - i.e. particles made of minerals (geological detritus) or organic matter (biological detritus) - that have been accumulated or deposited at Earth's surface. Sedimentation is any process that causes these particles to settle in place. Geological detritus originates from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus is formed by bodies and parts (mainly shells) of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies (marine snow). Sedimentation may also occur when dissolved minerals precipitate from water solution.
(A) Classification based on origin
Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: (1) clastic sedimentary rocks, (2) biochemical (biogenic) sedimentary rocks, (3) chemical sedimentary rocks, and (4) a fourth category for "other" sedimentary rocks formed by impacts, volcanism, and other minor processes
(1) Clastic sedimentary rocks
Clastic sedimentary rocks are composed of rock fragments (clasts) that have been cemented together. The clasts are commonly individual grains of quartz, feldspar, clay minerals, or mica. However, any type of mineral may be present. Clasts may also be lithic fragments composed of more than one mineral. Clastic sedimentary rocks are subdivided according to the dominant particle size.
Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel (>2 mm diameter), sand (0.06 to 2 mm diameter), and mud (60 μm diameter). Mud is further divided into silt (60 to 4 μm diameter) and clay (4 μm diameter).
The classification of clastic sedimentary rocks parallels this scheme; (1) conglomerates and breccias are made mostly of gravel, (2) sandstones are made mostly of sand, and (3) mudrocks are made mostly of mud. This tripartite subdivision is mirrored by the broad categories of rudites, arenites, and lutites, respectively, in older literature. The subdivision of these three broad categories is based on differences in clast shape (conglomerates and breccias), composition (sandstones), or grain size or texture (mudrocks).
(2) Biochemical sedimentary rocks
Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include:
- Most types of limestone are formed from the calcareous skeletons of organisms such as corals, mollusks, and foraminifera.
- Coal, formed from vegetation that has removed carbon from the atmosphere and combined it with other elements to build their tissue, this vegetation gets compressed by overlying sediments and undergoes chemical transformation.
- Deposits of chert formed from the accumulation of siliceous skeletons of microscopic organisms such as radiolaria and diatoms.
(3) Chemical sedimentary rocks
Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate. Common chemical sedimentary rocks include oolitic limestone and rocks composed of evaporite minerals, such as halite (rock salt), sylvite, baryte and gypsum.
(4) Other sedimentary rocks
This fourth miscellaneous category includes volcanic tuff and volcanic breccias formed by deposition and later cementation of lava fragments erupted by volcanoes, and impact breccias formed after impact events.
(B) Formation Processes of Sedimentary Rocks
The formation of sedimentary rocks involves several key processes:
- Weathering: Weathering breaks down existing rocks into smaller particles via chemical and physical processes. Freezing and thawing, salt, water, thermal stress, acid, and the action of plants and animals all alter rocks.
- Erosion: This involves the transportation of weathered products away from their physical location, by rock falls, wind, running water, gravity, or landslides.
- Transportation: This is the movement of these particles by wind, water, or ice.
- Deposition: Deposition is the settling of particles in new locations. Compaction and Cementation (Lithification or Diagenesis): Particles gradually accumulate and bind together over time.
- Precipitation: The deposited sediments then form rocks and minerals from chemicals that precipitate from water. As the water dries up, it leaves behind mineral deposits.
- Lithification (Diagenesis): Clay, sand, and other sediments on the bottom of the water bodies become sedimentary rock when slowly compacted into rocks from the weight of overlying sediments. Lithification releases moisture, thus reducing the pore space of the sediments, making the sedimentary rock compact. This process is also called compaction.
- Cementation: Finally, salt crystals glue the layers together to form a compact sedimentary rock.
Didcot #16 - Ladygrove Lakes
Ladygrove Lakes is a wildlife area with two fishing lakes, and is located in Didcot, Oxfordshire. The lakes are most noted for Carp/Specimen and Coarse fishing. Barbel, Bream and Common Carp are amongst the key species on offer at the venue. This is a Day Ticket fishery.
The geology around Ladygrove Lakes in Didcot primarily consists of Jurassic and Cretaceous clays, with an underlying layer of Upper Greensand. This area is characterized by undulating lowland farmland and a rolling landscape. The lakes themselves are likely man-made, potentially formed as part of the Didcot Garden Town development, and situated on this clay and greensand geology.
Additionally, this area is mapped as Gault Formation, a sedimentary bedrock formed during the Cretaceous period (BGS 2020). The bedrock is overlain by superficial head deposits which are recorded across the majority of the site.
Information Sources:
https://www.fishery.co.uk/ladygrove-lakes
https://eprints.oxfordarchaeology.com/5858/1/OAE_Report2454_XOXLAG20_FINAL_LR.pdf
https://en.wikipedia.org/wiki/Sedimentary_rock#Classification_based_on_composition
https://sciencenotes.org/sedimentary-rocks/
https://www.sciencefacts.net/sedimentary-rocks.html
About this EarthCache
This is our first EarthCache, so any comments and suggestions are welcome.
In order to log this EarthCache, please answer the following questions and send your answers to me via GeoCaching messaging services or by email:
(1) At the published coordinates, you will see two lakes, one on each side. Please look around and observe the sediments on the floor. Which above sedimentary rock category do you think these sediments belong to?
(2) Based on your observations, which formation process(es) do you think lead to the sediements you see here?
(3) Please post a photo of either yourself, or your GPS device, or your geocaching name on a screen/paper at GZ with one of the fishery lakes, within your finding log.
Please note, logs which do not meet the above logging requirements may be deleted without notice.
We wish you enjoy the beautiful views of the Ladygrove Lakes!
Thank you and enjoy GeoCaching!
FTF: Congratulations to Daffodilbob for their FTF on 20 August 2025!
