Tillite Identification Guide

What Tillite Looks Like

Tillite is the hardened, ancient equivalent of glacial till — a sedimentary rock deposited directly by ice. Its defining trait is that it is poorly sorted: pebbles, cobbles, and even boulders of mixed rock types float chaotically in a fine-grained mud or sand matrix.

• Color: dull gray, greenish-gray, brown, or reddish, depending on matrix
• Luster: dull, earthy
• Transparency: opaque
• Form: massive, structureless rock with embedded clasts of many sizes and compositions

Step-by-Step Field ID Checklist

1. Look for extremely poor sorting — clay-sized matrix holding clasts from sand grains up to boulders, with no layering.
2. Check that clasts are a mixture of rock types (granite, quartzite, limestone, etc.), implying long ice transport.
3. Examine pebble surfaces with a hand lens for glacial striations (parallel scratches) or faceted, flat-iron shapes.
4. Confirm clasts are randomly oriented, not aligned by water current.
5. Note the lack of bedding, cross-bedding, or graded layers.

Key Diagnostic Tests

• Hardness: variable — depends on clasts and cement; the matrix is often hard and well-cemented.
• Acid test: drip dilute HCl; fizzing indicates a calcareous matrix or limestone clasts (not diagnostic alone).
• Texture: the matrix-supported, unstratified fabric is the single most diagnostic feature.
• Striations: scratched, polished, or bullet-shaped clasts strongly confirm glacial origin.

Common Look-Alikes

• Conglomerate: clasts are rounded by water and the rock is usually better sorted and clast-supported, often bedded — tillite clasts are angular to subangular and unsorted.
• Breccia: angular clasts but typically of one local source (fault or scree), without the wide size range or striations.
• Diamictite: the general term for any unsorted mixed-clast rock; tillite is specifically the glacial kind — striated clasts confirm ice.
• Debris-flow/mudflow deposits: can mimic tillite, but lack striated, faceted clasts and often show flow structures.

Striated, far-traveled, mixed clasts in an unsorted muddy matrix point to tillite.

Where It Is Found

Tillites mark ancient ice ages: the Precambrian (e.g., the Gowganda Formation, Canada; 'Snowball Earth' diamictites), the Late Paleozoic glaciation of Gondwana (the Dwyka tillite of South Africa, India, Australia, South America), and Pleistocene-aged tills worldwide that are only weakly lithified.