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)
Fig. 34. Conglomerate Fig. 35. Breccia
Most sedimentary rocks are formed from
A. eroded rocks
B. eroded particles
C. eroded sand
The size of particles in most sedimentary rocks
A. bigger than sand grain
B. No bigger than sand grain
C. the same as sand grain
Geologists divide particles by size into
A. two groups
B. three groups
C. four groups
Lutites are
A. Medium-grained
B. coarse-grained
C. fine-grained
An example of arenites are
A. sandstones
B. Greywacke
C. siltstone
Fine-grained rocks are
A. mudstone, shale, orthoquartzite
B. mudstone, shale, siltstone
C. shale, arkose, graywacke
Rudites can be consolidated into natural concretes called
A. conglomerates and breccias
B. clasts and conglomerates
C. breccias and clasts
Conglomerates is a Latin word which means
A. coarse
B. rubble
C. lumped together
Conglomerates accumulate in
A. shallow coastal waters
B. shallow lakes
C. shallow coast lines
Conglomerates contain
A. rounded fragments
B. coarse debris
C. poorly sorted fragments
Breccias is usually embodies in
A. natural cement-sand
B. solid rock
C. clay-rich matrix
Tillites are different from conglomerates and breccias because
A. clasts with slightly rounded edges
B. ice-eroded debris
C. poorly sorted fragments
3.2.2 Read the text once more and fill in the charts with the necessary information from part 1 and part 2. Sedimentary rocks
Composition |
Size |
Types |
Features (what?) |
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