Introduction
Welcome to Agios Nikolaos, in eastern Crete! Here, you can observe an outcrop of conglomerates, witnesses to a turbulent geological history. These sedimentary rocks were formed several million years ago, during a period when Crete experienced phases of tectonic uplift and intense erosion.
The aim of this EarthCache is to help you discover this site and understand what a conglomerate is, how it forms, and the visual clues that can help identify its origin.
Reminder about EarthCaches: There is no container to find or logbook to fill out. You simply need to visit the location, answer the questions, and send me your answers.
To validate the Earthcache
To validate this EarthCache, you must correctly answer the following questions and include in your log a photo of yourself, your GPS, or your username at the cache waypoint. This photo will help confirm the “found” status in case of incorrect answers. Important: do not show your answers in the photo.
Please send your answers via my profile or through the geocaching.com messaging system—do not include them in your log. Logs submitted without answers will be deleted.
You may log the cache as “found” without waiting for my confirmation; I will contact you if there’s an issue.
Feel free to read the description text. You’ll likely need it to answer some of the questions.
Question 1 : What is the average size of the pebbles (in cm)? Are they rather rounded or angular in shape?
Question 2 : What is the dominant color of the pebbles? And what about the matrix?
Question 3 : "Can you identify the most common type of rock among the pebbles (limestone, flint, other)?"
Question 4 : Based on your observations, is it a polymictic conglomerate or an oligomictic conglomerate ? What is the type of matrix ? Please justify your answer.
Question 5 : Based on the size and shape of the pebbles, suggest a probable depositional environment (river, beach, alluvial fan, etc.). Explain your reasoning.
🧭 What is a conglomerate?
A conglomerate is a clastic sedimentary rock composed of clasts (pebbles, blocks) larger than 2 mm, bound together by a finer matrix (sand, silt) and a natural cement (often calcitic).
The coarse fragments must make up at least 10% of the rock, and their size must be greater than 2 millimeters (class: rudites).
When the fragments are angular, the rock is called a breccia; when they are rounded (pebbles), it is called a puddingstone (or conglomerate). However, there can be conglomerates where both angular and rounded elements coexist (these are called tillites).
🧬 How does a conglomerate form?
The formation of a conglomerate follows several steps:
- Erosion: Nearby reliefs are attacked by water, wind, or ice, producing rock fragments.
- Transport: The pebbles are carried by rivers or streams. Their rounded shape indicates prolonged transport in a high-energy environment.
- Deposition: When the energy decreases (e.g., alluvial fan, floodplain), the pebbles are deposited in layers.
- Diagenesis: Over time, pressure and the circulation of carbonate-rich fluids cement the pebbles together, forming the rock.
🧭 Classification of Conglomerates
Conglomerates are classified according to several criteria:
- According to the nature of the pebbles:
- Polymictic: contains several types of rocks (e.g., limestone + flint).
- Oligomictic: dominated by a single rock type (e.g., only limestone).
- According to the matrix:
- Orthoconglomerate: pebbles are well cemented, with little matrix.
- Paraconglomerate: abundant matrix, with dispersed pebbles.
- According to origin:
- Fluvial: formed in fast-flowing rivers.
- Alluvial: from alluvial fans.
- Marine: deposited on beaches or in coastal areas.
- Glacial (tillite): pebbles transported by glaciers.
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