Tillite
Tillite is lithified glacial till: a poorly sorted rock of clasts of every size in a fine matrix, deposited directly by glacial ice. The more general name for any such unsorted mixture of mud and gravel is diamictite.
Tillite is lithified glacial till: a poorly sorted rock of clasts of every size in a fine matrix, deposited directly by glacial ice. The more general name for any such unsorted mixture of mud and gravel is diamictite.
🪨 Geology Guide Specimen: Open Tillite (Diamictite) in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 6 subgroups of Sedimentary, and test your eye in LithoClade.
Glacial Signatures: How Do You Recognize Tillite?
Glaciers carry and grind rock of all sizes and drop it unsorted, so the pebbles and boulders float in a muddy matrix with no layering. Many clasts are faceted and scratched with parallel striations from being dragged along the ice base, and in layered mud, isolated "dropstones" show where rocks melted out of floating ice.
Deep Time: What Do Tillites Show?
Late Paleozoic tillites in Africa, South America, India and Australia helped convince early geologists that these continents had once been joined under a south polar ice sheet. Neoproterozoic tillites that formed near the equator support the Snowball Earth hypothesis, in which ice sheets reached low latitudes.
Caution: Are All Diamictites Glacial?
Debris flows, landslides and meteorite impacts can also produce unsorted deposits, so geologists look for supporting evidence such as striated pavements, dropstones and the regional setting before calling a diamictite glacial. See ice sheets for the modern record.
Tillite at a Glance
| Property | Value |
|---|---|
| Type / variety | Glacial Dropstone Diamictite Tillite |
| Composition | Striated Cobbles & Pebbles (>2 mm) Floating in Dark Rock-Flour Matrix |
| Geology Guide group | Sedimentary Rocks (Clastic, Chemical & Biogenic) |
| Rock Classification | Coarse Clastic (Rudites) |
| Mohs hardness | Mohs 5–6.5 |
| Texture / habit | Matrix-supported, completely unsorted clay-to-boulder clasts |
| Density | 2.60–2.75 g/cm³ |
| Lustre and cleavage | Faceted, ice-scratched pebbles suspended in dark grey mudrock |
| Geological setting | Continental ice sheets and glaciomarine dropstone basins |
| Diagnostic feature (Geology Guide) | Lithified glacial deposit where unsorted, ice-scratched pebbles and boulders (>2 mm) float randomly inside a dark, fine-grained 'rock-flour' matrix without water sorting. |
| Also called | Lithified till, a type of diamictite |
| Key evidence | Striated clasts and dropstones |