(a) What are landslides and avalanches? Discuss the causes and effects of landslides. Enumerate the remedial measures to prevent…

(a) Landslides and avalanches: causes, effects and remedial measures (20 marks)

Definitions. A landslide is the downslope movement of rock, debris or earth under gravity, as falls, slides, flows or topples. An avalanche is the rapid flow of a mass of snow and ice, often with rock debris, down a mountain slope.

Causes.

• Natural: steep slopes; weak, weathered or clay-rich rocks; unfavourable structure, with bedding, joints or faults dipping out of the slope; heavy rainfall and snowmelt, which raise pore pressure and reduce shear strength; earthquakes and volcanic activity; undercutting by rivers or the sea; freeze–thaw.
• Human: road and railway cuts, quarrying and blasting, deforestation, overloading of slopes, leaking irrigation and poor drainage.

Effects. The effects are loss of life, houses, roads, railways and hydropower works, blocking of roads and rivers, and burial of farmland. Landslide dams can burst as floods, and slides also raise siltation and increase the erosion of soil. Examples are Malin (2014), Kedarnath (2013) and Wayanad (2024), and avalanches endanger Himalayan roads and army posts.

Remedial and preventive measures.

• Hazard zonation mapping and land-use regulation, avoiding unstable slopes.
• Slope geometry: benching, flattening and removal of loose material.
• Retaining walls, gabions, rock bolts and anchors, shotcrete and wire mesh.
• Surface and sub-surface drainage, such as horizontal drains and catch drains.
• Afforestation and bioengineering (vetiver, grass), and check dams.
• Monitoring and early warning by rainfall thresholds, inclinometers and InSAR.
• For avalanches, supporting structures, snow sheds and galleries, deflecting walls and controlled blasting.

(b) Methods of palaeogeographic reconstruction (15 marks)

Palaeogeography reconstructs the ancient distribution of land, sea, climate and relief by combining several methods.

• Sedimentological: lithofacies and facies analysis (Walther's law) identify environments; cross-bedding and other structures give palaeocurrent direction and source area; isopach and lithofacies maps show basin shape and depth.
• Palaeontological: fossil assemblages show environment (shallow or deep, marine or freshwater), and faunal and floral provinces show connections between land masses. Index fossils correlate the same time across areas.
• Palaeomagnetic: remanent magnetisation gives the palaeolatitude and orientation, and apparent polar wander paths follow plate motion.
• Palaeoclimatic: tillites and striated pavements (cold), coal (humid), evaporites and aeolian sands (arid), bauxite and laterite (hot humid) and oxygen isotopes.
• Plate-tectonic: fitting of continental margins, matching of geology and orogenic belts, sea-floor magnetic anomalies and hotspot tracks.

Example. The Permian Gondwana reconstruction: the same Glossopteris flora and Mesosaurus in India, Africa, South America, Australia and Antarctica, the Talchir tillite, matching Gondwana coal basins and a fit at the continental slope. Palaeomagnetism of the Deccan Traps shows India at about 30°S at 66 Ma, drifting north to collide with Asia.

(c) Siwalik Group: classification, palaeogeography and fossils (15 marks)

Classification. The Siwalik Group is a Neogene to early Quaternary molasse, up to about 5–6 km thick, along the Himalayan foothills between the Main Boundary Thrust and the Himalayan Frontal Thrust. It is divided into three.

• Lower Siwalik (Kamlial and Chinji Formations; Miocene): fine sandstone and red and purple clay.
• Middle Siwalik (Nagri and Dhok Pathan Formations; Late Miocene to Pliocene): coarse, massive "salt and pepper" sandstone with clay.
• Upper Siwalik (Tatrot and Pinjor Formations; Pliocene to Pleistocene): conglomerate and sandstone, capped by the Boulder Conglomerate.

Palaeogeography. It is a foreland basin, formed in front of the rising Himalaya, filled by large rivers with braided and meandering channels and floodplains. The coarsening upwards, from clay to boulder conglomerate, records the increasing uplift and nearness of the source.

Fossils. The mammal fauna is rich: the apes Sivapithecus and Gigantopithecus, the giraffid Sivatherium, proboscideans (Stegodon, Deinotherium), the three-toed horse Hipparion, rhinoceros, hippopotamus (Hexaprotodon), bovids, and reptiles such as crocodile and Gavialis, tortoise and fish. The fauna indicates warm, seasonal savannah and grassland with rivers.