Fig.33 Ferruginous sandstone

Date: 2015-10-07; view: 588.

Part 2

Rudites (from the Latin rudis - coarse) are clasts (rock fragments) coarser than a sand grain. Mixes with finer particles, rudites can be consolidated into natural concretes called conglomerates and breccias.

Conglomerates are named from the Latin for “lumped together”. They contain rounded fragments – pebbles, cobbles and boulders – and often represent waterborne and watersorted remnants of eroded mountain ranges or retreating rocky coasts. They accumulate along mountain fronts, in shallow coastal waters, and elsewhere; becoming mixed with sand, then bound by natural cement. How clasts in a conglomerate lie sorted, packed and graded offers clues to how or where it was laid down. The thickest masses of conglomerate – as in the Siwalik Formation of the Himalayas' foothills – mark the aftermath of an orogeny.

Breccias (from the Italian for “rubble”) are rocks containing sharp-edged, unworn, usually poorly sorted fragments, often embedded in a clay-rich matrix. Breccias form usually near their place of origin; their clasts have not been carried far enough to suffer rounding by abrasion. Many breccias originate in talus, deserts, mudslides, faulting, meteorite impact, or shrinkage of evaporite beds.

Authorities tend to separate conglomerates and breccias from tillites – poorly sorted, ice-eroded, ice-borne debris consolidated into solid rock. Many tillite clasts are faceted, with slightly rounded edges. Ancient tillites occur in South America, Africa, India and Australia.

(David Lambert “The Field Guide to Geology” 1988, Cambridge University Press)