Rudaceous Rocks Examples (Characteristics and Types)

In this article, you’ll get to know some commonly known rudaceous rocks, as well as information about rudaceous rocks to better understand them.

Rudaceous Rocks Examples (Characteristics and Types)

Rudaceous rocks are abundant on the Earth’s surface, and they are often mixed with other types of rocks.

These types can be hard to identify unless they contain recognizable minerals such as granite, gneiss, basalt, or spongy quartz.

The word “rudaceous” originates from the ancient Latin word “rudes,” which means “coarsely broken.”

Referred to as “clastic sediments” because they are natural deposits from rock fragments eroded in the past and deposited by wind or water in layers to form sediments.

It has been opined that there is no fundamental difference between a fragment of sedimentary rock and a fragment of igneous or metamorphic rock.

These sedimentary rocks are formed by sediment deposition and successive compaction, cementation, or hardening.

When a sediment concentration is deposited by water or wind, it is called “clastic sediments” because the fragments of rocks are deposited without chemical precipitation.

Rudaceous rocks have the appearance of being poorly sorted. They can be identified as:

Conglomerate Rocks

All conglomerates have rounded pebbles and stones in varying sizes (from 1 mm to greater than 3 cm). The different types of Conglomerate Rocks include:

Bedrock conglomerate

This is a coarse-grained type of sedimentary rock in which the individual pebbles and boulders are not large enough to be called cobble or boulder conglomerates.

Instead, these reflect the size class seen in the picture below. The sizes range from 20 mm to 200mm (about an inch to 8 inches across).

That is considered a coarse-grained rock because of its size, but it also has other characteristics typical for Conglomerates. Examples include:

This rock can be found as a layer beneath newer sediments such as sandstone or shale.

You can find examples of bedrock conglomerates in the Grand Canyon and throughout the Southwest region of the United States.

Sandstone/shales conglomerate

These two separate rock types form when different-sized grains are carried and deposited.

For example, sandstones form in areas where the high mineral content is deposited in a river or other water bodies.

Shales form in areas where sediment-laden with mineral matter is being deposited, for example, in marine environments.

Sandstones typically form much faster than shales due to the presence of organic matter at the bottom of bodies of water that cause clasts to become coated with a thick biological film.

This biological film can be formed by plant and filamentous organisms, microorganisms such as cyanobacteria and algae, or accidental contamination from sewage seepage during heavy rainfalls onto mudflats and beaches.

For example, the sequence of fossiliferous strata in the Ypres beds at Ypres, Belgium, is a sequence of sandstones with increasing thickness and decreasing local fossils and clasts; a clear indication was obtained almost entirely from erosion of mudflats.

Shales typically form much slower than sandstones due to their organic content and more clay minerals.

Opaline conglomerate

These rocks are rounded pebbles and larger stones cemented together by fine-grained sand. They look similar to sandstone, but they offer a different texture.

They evolved from cobble conglomerates in the same way you would form a beach by moving large rocks to form the waves.

The pebbles formed water currents into cobble conglomerates, while the larger stones represent boulders that have fallen into the water during floods.

Cobble conglomerate/conglomerate

These rocks are composed of rounded and angular pebbles cemented together by finer sediment, forming a concrete-like matrix.

These conglomerates typically represent rapid deposition in streams or flash floods today and usually contain cobbles to boulder size stones up to one meter across.


Granite is both felsic (silicate rock) and mafic (silicate rock with a lot of magnesium).

That means it has crystals in the mineral structure cut by the six basic crystal shapes called hexagonal and rhombic-shaped crystals.

When exposed to high temperatures, granite may develop into an opaque rock containing quartz, feldspars, and micas.

These granites can contain mafic minerals that result in exciting textures and banding (a mineral with lines of varying mineral composition).


Breccias are rocks formed by a unique process called lithification.

Lithification is when loose particles in or under the Earth become cemented together in hard rock.

These particles come from things such as soil, dirt, and sand.


Breccias can be classified into three categories: coherent breccias, clastic breccias and non-brecciated masses.

Coherent breccias are the most common and contain angular material fragments that do not vary much in size.

This breccia type is usually formed from fine to coarse broken rock particles.

Clastic breccias are formed from angular pebbles and boulders. They are big rocks broken into pieces (crushing, abrasion).

This breccia type is usually composed of larger fragments than coherent and can have curved fragments.

Non-brecciated masses are blocks or masses that have not been lithified or cemented together by any rock material.

Breccias can be made from minerals and rocks such as quartz, feldspar, mica, and other ferromagnesium silicates.

Breccias are also formed from many different types of minerals.

Quartz is the most common mineral to be found in breccia.

Due to the shape and size of quartz grains, they tend to line up very uniformly.

These grains do not vary in size or shape; hence quartz creates a very uniform rock type when lithified.

Most breccias are mixed with igneous rock.

That means they contain fragments from molten lava or magma (volcanic rocks).

Many igneous rocks can be found inside and outside of breccias, such as granite, trachyte, and basalt.

Although many different types of rocks and minerals can be found in a breccia, they all tend to be very similar.

The rocks and minerals they contain range from round, angular, and angular-shaped. The sizes of their grains also range from small to large.


Breccias can vary a lot depending on where they were formed.

One must understand what type of rocks, soil, or minerals were in the breccia.

That can help determine if it is a coherent and clastic breccia, mass, or non-brecciated rock.

Coherent breccias

They are widespread and can be found everywhere in the world.

They can be made up of many minerals and rocks, such as quartz, feldspar, mica, and other ferromagnesium silicates.

Breccias are also formed from many different types of minerals.

Quartz is usually preferred over other minerals because it is less chemically reactive than feldspar or mica.

Therefore it does not break as easily when lithified or cemented together as a coherent breccia.

Coastal Clastic Breccia

It refers to a type of rock that forms due to sedimentary processes.

Coastal clastic breccia can be formed from many different sediments depending on the environment in which it is deposited and the sediment supporting it.

There are two main types of Coastal Clastic Breccias: Cohesive and Non-Cohesive.


These are breccias that have been cemented together by another material.

They usually contain many minerals, such as quartz, feldspar, and mica.

That gives them their name cohesive because of how strongly they hold together and the minerals that make up the cementing material.


These are not cemented together by a material, they are only held together by gravity or bound by spaces between other sedimentary rock layers.

These are very different from cohesive breccias because they contain only a few minerals, which means that they might not be able to bind to each other strongly enough to keep themselves intact.

There are two types of Rudaceous Rocks- breccias and Conglomerate rocks, and they all have different physical characteristics and appearances, and chemical compositions.

You can find these rocks in volcanic areas or worldwide scattered on the ground from thousands or millions of years ago when they were once hot and then cooled off in landmasses called Plates.

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Rudaceous Rocks Examples