Geomorphology

1. Classification of Geomorphic Processes

Geomorphic processes are all physical and chemical changes that effect a modification of the Earth's surficial landforms. They are broadly bifurcated based on their source of energy:

2. The Concept of Gradation (পর্যায়ন)

The term Gradation was introduced by American geologists Grove Karl Gilbert (1877) and refined by T.C. Chamberlin and R.D. Salisbury (1904). Gradation is the continuous leveling of the Earth's surface through the dual balancing operations of Degradation and Aggradation:

3. Base Level of Erosion (ক্ষয়ের শেষ সীমা)

The concept of Base Level of Erosion was formulated in 1875 by the American geologist John Wesley Powell during his seminal explorations of the Grand Canyon of the Colorado River:

• Definition: The lowest limit to which a stream or subaerial erosional agent can downcut and degrade its valley floor. It represents the theoretical plane below which dry land cannot be eroded by running water.
• Ultimate Base Level (চরম ক্ষয়সীমা): Mean Sea Level (MSL) represents the global ultimate base level, since rivers cannot cut their valleys below the level of the ocean into which they discharge.
• Temporary or Local Base Level (সাময়িক ক্ষয়সীমা): Inland lakes, resistant rock sills, dams, waterfalls, or tributary junctions with main rivers act as local base levels for upstream river segments until the barrier is breached or silted up.

1. Subsurface Vertical Profile and the Water Table

Meteoric water (precipitation) infiltrates and percolates through rock pores and fractures under gravity, establishing distinct hydrogeological zones:

2. Aquifer Typology and Geological Formations

The capacity of subterranean geological strata to store and transmit groundwater depends on two properties: Porosity (percentage of void volume: $n = V_v / V_t imes 100$) and Permeability (interconnectedness of pores allowing fluid flow):

• Aquifer (জলবাহী স্তর): A permeable, water-bearing geological stratum capable of yielding economic quantities of water (e.g., unconsolidated sand, gravel, porous sandstone).
• Aquiclude (জলরোধক স্তর): A porous rock stratum capable of absorbing water but incapable of transmitting it due to extremely minute pores (e.g., impermeable clay).
• Aquitard (জলমন্থর স্তর): A semi-pervious stratum that retards groundwater flow, transmitting water at very slow rates (e.g., sandy clay, silt).
• Aquifuge (জলাভেদ্য স্তর): An entirely impermeable and non-porous rock stratum containing no interconnected pores and transmitting zero water (e.g., unfractured solid granite or quartzite).

3. Springs: Mechanics and Classification

A Spring (প্রস্রবণ) is a natural concentrated outflow of groundwater emerging onto the surface of the Earth:

• Gravity Springs: Outflows driven by hydraulic gradient where the water table naturally intersects the ground surface (contact springs, fault-line springs).
• Artesian Springs & Wells: In a synclinal geological basin where a confined aquifer is tilted between impermeable strata, hydraulic pressure forces water to rise spontaneously without pumping (named after Artois province, France).
• Karst / Vauclusian Springs: Large subterranean rivers emerging from limestone massifs under hydrostatic head, named after Fontaine de Vaucluse in southern France.
• Thermal & Mineral Springs: Groundwater heated by subterranean geothermal magma chambers, rich in dissolved sulphur or minerals (e.g., Bakreshwar in Birbhum, West Bengal).
• Geysers: Intermittent thermal springs that periodically erupt violent boiling jets of water and steam through a restricted vent due to superheated subterranean steam expansion (e.g., Old Faithful in Yellowstone, USA).

1. Essential Prerequisites for Karst Development

Named after the classic limestone limestone plateau of Kras (Karst) along the Adriatic coast of Slovenia and Croatia, karst landforms develop only under strict environmental conditions:

• Lithological Purity: Thick bedded, pure calcium carbonate limestone ($CaCO_3 > 80\%$) or dolomite.
• Dense Structural Jointing: Limestone is relatively impermeable per se, but well-developed orthogonal joints, fissures, and bedding planes allow groundwater ingress.
• Entrenched Relief & Deep Water Table: Incised river valleys nearby maintain a low regional water table, enabling rapid vertical percolation and vigorous subterranean circulation.
• Adequate Precipitation: Humid temperate or tropical climate providing abundant rainfall charged with carbon dioxide.

2. Karst Chemical Solution Chemistry

Pure water dissolves limestone negligibly. Rainwater absorbs atmospheric and biogenic $CO_2$ from soil organic matter, forming weak carbonic acid ($H_2CO_3$), which converts insoluble calcium carbonate into highly soluble calcium bicarbonate:

3. Erosional Karst Landforms

Continuous chemical solution along joints and fissures carves an evolutionary sequence of distinctive surface depressions:

4. Depositional Karst Landforms (Speleothems)

Inside subterranean caverns, when dripping water containing dissolved $Ca(HCO_3)_2$ encounters cavern air, carbon dioxide escapes ($CO_2$ degassing) and moisture evaporates, precipitating insoluble calcium carbonate:

• Stalactite (স্ট্যালাকটাইট): Icicle-like calcite columns hanging downward from the ceiling of limestone caves, built up droplet by droplet around a central hollow capillary tube.
• Stalagmite (স্ট্যালাগমাটি): Blunt, rounded, broader calcite mounds growing upward from the cavern floor, formed by water dripping from the ceiling above. Lacks a central hollow tube.
• Cave Pillar / Column (গুহা স্তম্ভ): Formed when a descending stalactite and an ascending stalagmite coalesce into a continuous vertical pillar.
• Dripstone & Helictites: Flowstones, stone curtains, and helictites (irregular calcite structures growing sideways or upwards against gravity due to capillary action).

1. Wave Dynamics and Littoral Processes

Ocean waves generated by frictional drag of wind over open sea are the primary agents shaping coastlines:

• Oscillatory vs. Translatory Waves: In deep water, water particles move in circular orbits with negligible forward mass transport (oscillatory waves). As waves enter shallow shoals (depth < half wavelength), frictional drag at the seafloor causes wave velocity to decrease and wave height to increase until the wave steepens and breaks, forming a turbulent forward-moving sheet of water called a wave of translation (ব্রেকার / তরঙ্গ ভগ্ন).
• Swash and Backwash (সোয়াশ ও ব্যাকওয়াশ): Swash is the surging rush of turbulent seawater up the beach face following wave breaking. Backwash is the gravitational return flow of water down the beach face towards the sea.
• Constructive vs. Destructive Waves:
  • Constructive Waves (গঠনকারী তরঙ্গ): Low frequency (6–8 per minute), low wave height, powerful swash and weak backwash. Deposits shingle and sand on the beach.
  • Destructive Waves (বিনাশকারী তরঙ্গ): High frequency (10–14 per minute), steep high waves, weak swash and powerful backwash. Combines hydraulic scouring to strip beach sediment offshore.
• Longshore Drift (তীর বরাবর স্রোত): Oblique approach of waves to the coastline causes swash to carry sediment up the beach at an angle, while backwash pulls it straight down perpendicular to the shoreline under gravity, producing a continuous zig-zag transport of sand along the coast.

2. Coastal Landforms of Erosion

High-energy storm waves attack resistant rocky coastlines through hydraulic action, abrasion (corrasion), attrition, and chemical corrosion:

3. Coastal Landforms of Deposition

Constructive waves and longshore currents deposit sediment in sheltered bays, estuaries, and shallow coastal zones:

• Beach (সৈকতভূমি): A temporary accumulation of sand, shingle, and pebbles along the shoreline between high and low water marks, often featuring elevated storm ridges called berms.
• Spit & Hook (স্পিট ও হুক): A narrow ridge of sand or shingle projecting out from the mainland into an open bay or estuary, with one end attached to the land and the other terminating in open water. Curved by wave refraction at its outer tip, it forms a hook.
• Tombolo (টমবলো): A depositional ridge of sand or spit that joins an offshore island to the mainland (e.g., Chesil Beach connecting Isle of Portland to Dorset, UK).
• Offshore Barrier Bar & Lagoon (উপকূলীয় বাঁধ ও উপহ্রদ): A submerged or emergent sand ridge built parallel to the coast by breaking waves. When it encloses a shallow body of calm seawater between itself and the mainland, it creates a lagoon (e.g., Chilika Lake in Odisha).

4. Classification of Coastlines

Coastlines are classified based on eustatic sea-level fluctuations and tectonic movements into two primary categories:

1. William Morris Davis's Geographical Cycle (1899)

American geomorphologist William Morris Davis formulated the first comprehensive sequential model of landform development, famously stating: "Landscape is a function of structure, process, and stage"—known as the Trio of Davis (ডেভিসের ত্রয়ী):

• Core Axioms: (1) Rapid, brief, episodic crustal uplift of a landmass without significant erosion during uplift; (2) Long period of tectonic stability during which denudation operates uninterrupted until the landmass is worn down to base level.

2. Walther Penck's Morphological Analysis (1924)

German geologist Walther Penck rejected Davis's sequential stages and assumption of rapid uplift followed by static erosion. In his Morphological Analysis of Landforms (1924), Penck argued that landforms represent a continuous, dynamic competition between the rate of crustal uplift and the rate of denudation:

3. Interruption of the Cycle and Rejuvenation (পুনর্যৌবন লাভ)

An erosion cycle is rarely allowed to complete its course to a true peneplain. Any event that restores vigorous erosional energy to a mature or old-stage river is called Rejuvenation, initiating a new cycle and creating Polycyclic Relief (বহুচক্রীয় ভূমিরূপ):

• Causes of Rejuvenation: (1) Dynamic Rejuvenation (tectonic uplift of the landmass); (2) Eustatic Rejuvenation (worldwide drop in sea level, lowering base level); (3) Static Rejuvenation (increased river discharge from climatic wet shifts or river capture).
• Knick Point (নিক পয়েন্ট): A sharp break of slope in the longitudinal profile of a river where the new rejuvenated gradient meets the older, gentler gradient, marked by waterfalls and rapids.
• Paired River Terraces (যুগ্ম নদীমঞ্চ): Step-like abandoned remnants of former valley floors running parallel on both sides of a river, formed when the rejuvenated river rapidly incises into its own ancient floodplain.
• Incised Meanders & Valley-in-Valley: Rejuvenation forces an existing mature meandering stream to vertically entrench its winding course into solid bedrock (forming entrenched or ingrown meanders and nested valley profiles).

1. Genetic Classification of Drainage Systems

Classified based on the relationship between stream flow direction and the underlying geological structure and original regional slope:

2. Discordant / Unadjusted Drainage Systems

Drainage systems that cut across underlying geologic structures without exhibiting structural accord:

• Antecedent Drainage (পূর্ববর্তী নদী): A river that existed prior to the tectonic uplift of a mountain range. As the mountains rose across its path, the river maintained its established course by cutting down its channel at a rate equal to or exceeding the rate of tectonic uplift, carving stupendous transverse gorges (e.g., Indus, Brahmaputra, and Sutlej cutting across the rising Great Himalayas).
• Superimposed Drainage (অধ্যাপিত / অধ্যারোপিত নদী): A drainage pattern that was originally established on a younger overlying geological cover (sedimentary or lava cover). As erosion stripped away the surface layer, the stream entrenched its existing pattern directly into the completely discordant ancient basement structures below, preserving a pattern entirely unrelated to current structural rocks (e.g., Subarnarekha on the Chota Nagpur Plateau).

3. Geometric Drainage Patterns (নদী নকশা)

The plan view or spatial geometric arrangement formed by a master river and all its tributaries reflects underlying lithology, rock hardness, and fracture systems:

1. Factors of Soil Formation: Dokuchaev & Hans Jenny

Russian scientist V.V. Dokuchaev (the father of modern pedology) established that soil is an independent natural body formed by environmental factors. In 1941, Hans Jenny formulated the classic mathematical function:

• Active Factors (সক্রিয় নিয়ন্ত্রক): Climate (temperature and precipitation driving leaching rates) and Organisms (microbes, vegetation, burrowing fauna supplying humus).
• Passive Factors (নিষ্ক্রিয় নিয়ন্ত্রক): Parent Rock (determines mineralogy and initial texture), Topography/Relief (slope controls drainage and profile thickness), and Time (maturity of horizon differentiation).

2. The Master Soil Profile (মৃত্তিকা পরিলেখ ও স্তরসমূহ)

A vertical section through the soil exposing all successive horizontal layers (horizons) from the ground surface down to unweathered bedrock:

3. Zonal Pedogenic Processes

Specific biochemical processes governed by macroclimate and natural vegetation that produce distinct soil types:

4. USDA Soil Taxonomy and Conservation

The United States Department of Agriculture (USDA) Soil Taxonomy classifies soils into 12 comprehensive orders based on diagnostic horizons:

• Entisols: Youngest, recently formed soils with no genetic horizon development (alluvium).
• Inceptisols: Embryonic soils with weak initial horizon development.
• Mollisols: Fertile grassland soils with thick, dark, organic-rich surface horizons.
• Oxisols: Intensely weathered, nutrient-depleted tropical soils rich in iron sesquioxides.
• Spodosols: Acidic forest soils with prominent bleached E-horizons and illuvial organic/aluminum B-horizons.
• Vertisols: Heavy, swelling-shrinking clay soils prone to deep cracking during dry spells (black cotton soils / Regur).
• Aridisols: Desert and dryland soils with low organic matter and high salt accumulation.