Sedimentary rocks Formed in seas and lakes Formation

Sedimentary rocks Formed in seas and lakes

Formation ¨ Rock materials ¨ Derived from weathering and erosion ¨ Are transported by river / wind ¨ And later deposited (sedimentation) ¨ Under the pressure of the overlying sediment ¨ These materials are hardened into solid rock

Characteristics of sedimentary rocks ¨ Non-crystalline ¨ Stratified (layered) ¨ Bedding planes between rock strata ¨ May contain fossil

Conglomerate

Characteristics ¨ Rock containing rounded pebbles ¨ they are cemented by fine groundmass ¨ It is more resistant ¨ Their outcrop across stream beds may produce waterfalls and rapids

Breccia

Characteristics ¨ The rock contains angular fragments

Breccia – Ap Chau

Folding in sedimentary rocks

Ammonite fossil

Sandstone

Sandstone

Characteristics ¨ Sand grains are cemented ¨ Deposited farther away from the coast ¨ Less resistant than conglomerate and breccia

Siltstone

Stratification in shale e. g. Tung Ping Chau

Siltstone layer being eroded to form a sea arch, Tai O.

Characteristics ¨ Very fine sediment of silt and mud is deposited and cemented together on sea floor far away from coast ¨ Made up of numerous thin layers (stratified structure) ¨ It is not resistant to weathering and erosion

Cross-bedding in siltstone

Faulting in siltstone

Igneous rocks - Volcanic rocks Ash, pyroclasts and lava flows

Flowlines in rhyolite

Columnar jointing due to cooling of lava - rhyolite

Coarse tuff

Characteristics ¨ Ash and pyroclastic materials ejected during volcanic eruption ¨ Presence of volcanic bombs

Volcanic bombs

Igneous rocks - Hypabyssal rocks In sills and dykes

Dolerite dyke

Dolerite

Feldspar porphyry

Igneous rocks In batholith

3 sets of joints in granite

Granite

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