Home Sedimantery Rocks Clastic Sedimentary Rocks Conglomerate



Conglomerate is a clastic sedimentary rock that shaped from rounded gravel and boulder sized clasts cemented or in a matrix supperted. The rounding of the clasts show that rocks have been transported a long way from their source or on a seaside tide to wave movement. The clast cement is usually calcite, silica or iron oxide but the matrix can consist only of the cementing cloth, however can also include sand and / or silt sized clasts cemented together the various coarser clasts.

Class: Conglomerate may be divided into large lessons:

Texture: Clastic (coarse-grained).

Grain size: > 2mm; Clasts easily visible to the naked eye, should be identifiable.

Hardness: Soft to hard, dependent on clast composition and strength of cement.

Colour: variable, dependent on clast and matrix composition.

Clasts: variable, but generally harder rock types and / or minerals dominate.

Other features: Clasts generally smooth to touch, matrix variable.

Classification of Conglomerate

Conglomerates named and classifield by the

  1. Type and amount of matrix present
  2. Composition of gravel-size clasts they contain
  3. Size range of gravel-size clasts present

A sedimentary rock consisting mainly of gravel is first named according to the roundness of the gravel. If the gravel clasts that form it are well-rounded to subrounded, to a large extent, it is a conglomerate. If the pebble clips forming it are largely angular, it is a breccia. Such breccias may be called sedimentary breccias to distinguish them from other breccia types.

  1. The amount and chemical composition of the matrix. If the clasts do not touch each other (lots of matrix), the rock is paraconglomerate. Rock in which the clasts touch each other is called orthoconglomerate.
  2. The composition of the clasts. If all the clasts are the same type of rock or mineral), the rock is categorized as monomictic conglomerate. If the clasts are made up of two or more rocks or minerals, the rock is a polymictic conglomerate.
  3. The size of the clasts. Rock comprised of large clasts is cobble conglomerate. If the clasts are pebble-sized, the rock is called pebble conglomerate. If the clasts are small granules, the rock is called granule conglomerate.

The environment that deposited the material. Conglomerates may form from glacial, alluvial, fluvial, deepwater marine, or shallow marine environments.

Conglomerate Composition

Conglomerate can have a variety of compositions. As a clastic sedimentary rock, it can contain clasts of any rock material or weathering product that is washed downstream or down current. The rounded clasts of conglomerate can be mineral particles such as quartz, or they can be sedimentary, metamorphic, or igneous rock fragments. The matrix that binds the large clasts together can be a mixture of sand, mud, and chemical cement.


Conglomerate can be formed at an areas where strong water current exist like mountain down slope where water has enough current flow that it can carry the rock fragments above 2 millimetre. It can also be formed at beaches where water current is strong and rock fragments are available to be accumulated for forming conglomerate. Conglomerate is formed when large clast pebble or cobble size fragments transported and deposited than the finer grained fills the spaces in between the clast.

Where is it found

Conglomerates are deposited in various sedimentary environments.

Deepwater marine

In turbidites, the basal part of a bed is typically coarse-grained and sometimes conglomeratic. In this setting, conglomerates are normally very well sorted, well-rounded and often with a strong A-axis type imbrication of the clasts.

Shallow marine

Conglomerates are normally present at the base of sequences laid down during marine transgressions above an unconformity, and are known as basal conglomerates. They represent the position of the shoreline at a particular time and are diachronous.


Conglomerates deposited in fluvial environments are typically well rounded and well sorted. Clasts of this size are carried as bedload and only at times of high flow-rate. The maximum clast size decreases as the clasts are transported further due to attrition, so conglomerates are more characteristic of immature river systems. In the sediments deposited by mature rivers, conglomerates are generally confined to the basal part of a channel fill where they are known as pebble lags. Conglomerates deposited in a fluvial environment often have an AB-plane type imbrication.


Alluvial deposits form in areas of high relief and are typically coarse-grained. At mountain fronts individual alluvial fans merge to form braidplains and these two environments are associated with the thickest deposits of conglomerates. The bulk of conglomerates deposited in this setting are clast-supported with a strong AB-plane imbrication. Matrix-supported conglomerates, as a result of debris-flow deposition, are quite commonly associated with many alluvial fans. When such conglomerates accumulate within an alluvial fan, in rapidly eroding (e.g., desert) environments, the resulting rock unit is often called a fanglomerate.


Glaciers carry a lot of coarse-grained material and many glacial deposits are conglomeratic. Tillites, the sediments deposited directly by a glacier, are typically poorly sorted, matrix-supported conglomerates. The matrix is generally fine-grained, consisting of finely milled rock fragments. Waterlaid deposits associated with glaciers are often conglomeratic, forming structures such as eskers.

Characteristics and Properties

Conglomerate is a sedimentary rock that looks like concrete. It consists of large, rounded pebbles (clasts) cemented by a matrix made of calcite, iron oxide, or silica.

Conglomerate rock occurs where gravel can become rounded by traveling distances or being subjected to tumbling. Beaches, riverbeds, and glaciers can produce conglomerate.

The properties of conglomerate rock depend on its composition. It can be found in any color and may be either hard or soft.

Conglomerate can be used as a fill material for roads and construction. Hard rock may be cut and polished to make dimension stone.

Conglomerate Uses

Conglomerate has very few uses because of it not clean breakage and fine particles are unreliable. It can only be used as a crush where low performance material is wanted. Conglomerate has very few commercial uses. Its inability to break cleanly makes it a poor candidate for dimension stone, and its variable composition makes it a rock of unreliable physical strength and durability. Conglomerate can be crushed to make a fine aggregate that can be used where a low-performance material is suitable. Many conglomerates are colorful and attractive rocks, but they are only rarely used as an ornamental stone for interior use.

Analysis of conglomerate can sometimes be used as a prospecting tool. For example, most diamond deposits are hosted in kimberlite. If a conglomerate contains clasts of kimberlite, then the source of that kimberlite must be somewhere upstream.

Conglomerate and Breccia

Conglomerates and breccias are two sedimentary rocks close to each other, but differ significantly in the form of clasts. Clasts in the conglomerate are rounded or at least partially rounded, whereas the clast in the breccias have sharp corners. Sometimes sedimentary rocks contain a mixture of round and angled buckles. This type of rock can be called breccio-conglomerate.


  • Conglomerate is closely related to sandstone and displays many of the same types of sedimentary structures. Sandstone is a notably popular building material, used for things like flagstones and tile.
  • Conglomerate rocks are colorful and attractive; however, it is rarely used as ornamental stone for interior use because of its unreliable physical strength and durability.
  • Conglomerate has very few commercial uses, though it can be crushed to make a fine aggregate that can be used when a low-performance material is needed.
  • Conglomerate forms where sediments of rounded clasts at least two millimeters in diameter accumulate. Because of the large size of the clasts, it takes a very strong water current to transport and shape the rocks. As they tumble through the running water or moving waves, they form their rounded shape.
  • These rocks can be found in sedimentary rock sequences of all ages. They probably make up less than one percent by weight of all sedimentary rocks.
  • When the gravel clasts in a conglomerate are separated from each other and contain more matrix than clasts, it is called a paraconglomerate. When they are in contact with each other, it is called a orthoconglomerate.
  • Similar sedimentary rocks that are composed of large angular clasts are referred to as breccia. While a conglomerate is composed of rounded clasts, breccia is composed of broken rocks or minerals.
  • NASA’s Mars rover Curiosity discovered an outcrop of conglomerate on the surface of Mars in September 2012. This provided evidence to scientists that a stream once ran across the area where the rover was driving. The shape and sizes of the stones can offer clues to the distance and speed of the stream’s flow.


Cite this article as: Geology Science. (2019). Conglomerate. [online] Available at: http://geologyscience.com/rocks/sedimantery-rocks/conglomerate/ [9th December 2019 ]