Rock Identifier
Conglomerate (Conglomerate (Clastic Sedimentary Rock)) — sedimentary
sedimentary

Conglomerate

Conglomerate (Clastic Sedimentary Rock)

Clastic texture with rounded to sub-angular clasts (gravel-sized) embedded in a finer-grained matrix. Colors range from tan, brown, to grey depending on the cement and clast composition. Mohs hardness varies (6-7 for quartz clasts), luster is generally dull or earthy, and it lacks a specific crystal structure due to be…

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Physical properties

Clastic texture with rounded to sub-angular clasts (gravel-sized) embedded in a finer-grained matrix. Colors range from tan, brown, to grey depending on the cement and clast composition. Mohs hardness varies (6-7 for quartz clasts), luster is generally dull or earthy, and it lacks a specific crystal structure due to being a composite rock.

Formation & geological history

Formed by the accumulation of rounded water-worn pebbles and cobbles in high-energy environments like riverbeds, alluvial fans, or beaches. Over time, these sediments are buried and cemented together by minerals like silica, calcite, or iron oxide.

Uses & applications

Commonly used as a coarse aggregate in the construction industry for concrete and roads. Some colorful varieties are used as decorative stone or for lapidary work. It is also an important indicator for geologists identifying ancient river systems.

Geological facts

Conglomerate is often called 'puddingstone' because it resembles a fruit pudding. It is much more common than its sharp-edged relative, breccia, because constant water movement naturally rounds the jagged edges of rock fragments over distance.

Field identification & locations

Identify in the field by looking for large, rounded stones clearly stuck inside a finer sandy or silty matrix. Look for it in areas with ancient alluvial deposits or near current river systems and coastal cliffs.

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