Sedimentary Tillite & Glacial Drift
Tillite is a sedimentary rock formed from glacial till. Glacial till is poorly
sorted sediment with mixed grain sizes including clay to boulders. Larger particles
often show striations. When lithified, these sediments form a rock that, like
conglomerate, is a combination of fragments of many sizes, shapes, mineral composition
and rock type origins. In contrast to the rounded fragments seen in a conglomerate,
the fragments in Tillite, formed from direct glacial deposits and not from sediments
rounded by tumbling in running water, may have sharp angular edges and larger
clasts may be striated.
Tillite may contain partly rounded sediments if formed from fragments that were
reworked by glacial meltwater prior to deposition on an Outwash Plain.
The illustration below shows a retreating continental glacier. The Terminal
(End) Moraine is a depositional feature consisting of mixed weathered and eroded
rock fragments. Lithification of such glacial sediments would form sedimentary
Tillite.
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Glacial Drift - Unconsolidated, unstratified glacially
deposited sediments of mixed sizes, shape and composition. Fragments may be deposited directly by a glacier (called Till) or they may be deposited by glacial meltwater on an outwash plain (called Stratified Drift). Glacial Till in Exposed in Roadcut; Photograph taken in June 2003 by Daniel Mayer. |
| Tillite - Lithified Glacial Till, a sedimentary
rock formed from sediments of mixed sizes, shapes and composition. Tillite Bedrock Outcrop, North of Bruce Mines in Ontario, photo by L. Joseph, courtesy of and © Illinois State Geological Survey, USGS. |
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