Informative reading- rocks from fragments

3.2.1 Read the text (Part 1, Part 2) and for statements 1-12, choose the best answer: A, B, C or D. Then explain the words in bold.

Part 1

Most sedimentary rocks form from particles eroded from the rocks on land. Their main ingredients are clasts (rock fragments) of quartz, feldspar and clay minerals. These fragments range in size from microscopic grains to boulders.

More than 90% of all sedimentary rock contains particles no bigger than a sand grain. Many geologists classify such particles by size in two main groups. The (fine-grained) lutites with grains less than 0.06 mm diameter produce mudstone, siltstone and shale. The (medium-grained) arenites or sandstones with grains of 0.06-2 mm give arkose, graywacke and orthoquartzite. Here are brief descriptions of six fine- and medium- grained rocks.

1. Mudstone - solid rock made of clay minerals of less than 0.004mm diameter.

2. Shale - mudstone, siltstone or similar fine-grained rock of silt and clay split easily along its bedding planes. Shale accounts for more than 80% of all sedimentary rock.

3. Siltstone- rock formed of particles 0.004-0.06mm in diameter.

4. Orthoquartzite- a “clean” or pure arenite mainly made of quartz after other substances have been removed. (Arenites account for more than 10% of all sedimentary rock)

5. Arkose- an arenite rich in feldspar derived from gneiss or granite.

6. Graywacke- a muddy, often grayish sandstone with mixed-size particles including quartz, clay minerals and others.

Fig.33 Ferruginous sandstone

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)