Conglomerate vs Breccia
conglomerate and breccia are both coarse-grained sedimentary rocks made of fragments (clasts) cemented by a finer matrix, but they differ in clast shape: conglomerate has rounded, water-worn pebbles while breccia has sharp, angular pieces. conglomerate usually forms where transport and abrasion round clastsâthink rivers, beaches, and alluvial fansâso itâs often better sorted and feels âpebblyâ in the hand.
Breccia forms near the source of breakageâfault zones, landslides, karst collapse, volcanic or impact eventsâso itâs commonly poorly sorted with jigsaw-like fits between fragments. Both can be clast- or matrix-supported, with silica, calcite, or iron oxide cement.
For Ontario rockhounding, rounded-pebble conglomerates are common along Great Lakes shorelines and in glacial deposits, while breccias appear in faulted bedrock, carbonate terrains, volcanic belts, and impact structures; the same identification tipsâstart with clast shape, then confirm with sorting and contextâapply anywhere.
| Feature | Conglomerate | Breccia |
|---|---|---|
| Rock Type | Sedimentary | Sedimentary (sometimes volcanic) |
| Clast Shape | Rounded pebbles & cobbles | Angular, jagged fragments |
| Clast Size | > 2 mm (pebbles, cobbles, boulders) | > 2 mm (same size range as conglomerate) |
| Cement | Sand, silt, silica, or calcite | Clay, calcite, silica, or volcanic ash |
| Appearance | Looks like a âstone fruitcakeâ or puddingstone | Looks like a âjigsaw puzzleâ of broken rock |
| Formation | Fragments transported by water (river/shoreline) and rounded before cementing | Fragments broken in place by faulting, landslides, or eruptions, then quickly cemented |
| Durability | Usually well-cemented, but varies | Varies widely; some crumble if cement is soft |
| Rock Tumbling | Can polish well if silica-cemented, but often uneven due to mixed hardness | Polishes into colourful speckled stones (good with jasper or quartz clasts) |
| Where Found (Lake Ontario) | Glacial gravels, pebble beaches, puddingstone erratics | Less common; occasional erratics in mixed gravels |
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