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Landform Evolution

  • landform is a small to medium sized part of the surface of the earth
  • Several related landforms together make up landscapes. Each landform has its own physical shape, size, materials and is a result of the action of certain geomorphic processes and agents
  • Actions of most of the geomorphic processes and agents are slow, and hence the results take a long time to take shape.
  • Every landform has a beginning. Landforms once formed may change in their shape, size and nature slowly or fast due to continued action of geomorphic processes and agents

RUNNING WATER

  • In humid regions, which receive heavy rainfall running water is considered the most important of the geomorphic agents in bringing about the degradation of the land surface.
  • There are two components of running water.
    1.  overland flow on general land surface as a sheet.
    2. Linear flow as streams and rivers in valleys.
  • Most of the erosional landforms made by running water are associated with vigorous and youthful rivers flowing over steep gradients.
  • There may be depositional forms associated with streams flowing over steep slopes
  • After continuous erosion, down cutting becomes less dominant and lateral erosion of banks increases and as a consequence the hills and valleys are reduced to plains.
  • Overland flow causes sheet erosion
  • The divides between drainage basins are lowered until they are almost completely flattened leaving finally, a lowland of faint relief with some low resistant remnants called monadnocks

Stages of water flow.

Youth stage

  • V-shaped valleys with no
  • floodplains
  • Absence or less Meanders
  • Waterfalls and rapids

Mature

  • V shaped valley
  • NO waterfall and rapids

Old

  • Natural levees
  • Vast flood plains
  • Oxbow lakes
  • Meanders

EROSIONAL LANDFORMS

VALLEYS

  • Valleys start as small and narrow rills  The rills will gradually develop into long and wide gullies the gullies will further deepen, widen and lengthen to give rise to valleys
    • A gorge is almost equal in width at its top as well as its bottom. In contrast, a canyon is wider at its top than at its bottom
    • Valley types depend upon the type and structure of rocks in which they form.

PLUNGE POOLS

  • Large and deep holes at the base of waterfalls are called plunge pools.
    • Formed by the force of falling water.

ENTRENCHED MEANDERS

  • very deep and wide meanders that cut in hard Rocks are called incised or entrenched meanders
    • In the case of steep gradient streams, lateral erosion on the sides of the valleys is not much when compared to the streams flowing on low and gentle slopes
    • Because of active lateral erosion, streams flowing over gentle slopes, develop sinuous or meandering courses. It is common to find meandering courses over floodplains and delta plains where stream gradients are very gentle

RIVER TERRACES

  • River terraces are basically products of erosion as they result due to vertical erosion by the stream into its own depositional floodplain
    • The river terraces may occur at the same elevation on either side of the rivers in which case they are called paired terraces.

DEPOSITIONAL LANDFORMS

ALLUVIAL FANS

  • Alluvial fans are formed when streams flowing from higher levels break into foot slope plains of low gradient. Normally very coarse load is carried by streams flowing over mountain slopes.
    • This load becomes too heavy for the streams to be carried over and these depositions from alluvial fans.

DELTAS

  • The load carried by the rivers is dumped and spread into the sea. If this load is not carried away far into the sea or distributed along the coast, it spreads and Deltas are formed.
  • Unlike in alluvial fans, the deposits making up deltas are very well sorted with clear stratification

FLOOD PLAINS

  • Fine Sized materials like sand, silt and clay are carried by flowing water down the gentle slope. These materials are deposited to form flood plains.
    • river bed made of river deposits is the active floodplain. The floodplain above the bank is inactive floodplain.
- --Poinff 
levee -

MEANDERS

  • Meander is not a landform but is only a type of channel pattern. This is because of
  • propensity of water flowing over very gentle gradients to work laterally on the banks
  • unconsolidated nature of alluvial deposits making up the banks with many irregularities which can be used by water exerting pressure Laterally
  • coriolis force acting on the fluid water deflecting it like it deflects the wind.
  • When the gradient of the channel becomes extremely low, water flows leisurely and starts working laterally.
Inflection 
point 
b auk 
Point 
Point 
bars 
Slip-off bank' 
(Concave bank 
Slip-off bank 
RIGHT BANK 
LEFT BANK

GROUND WATER

  • After vertical penetration water moves to some depth, then

the water under the ground flows horizontally through the bedding planes

  • this downward and horizontal movement of water which causes the rocks to erode.
  • Pools, Sinkholes, Lapies and Limestone Pavements are erosional landforms
  • Stalactites hang as icicles of different diameters. Normally they are broad at their bases and taper towards the free ends showing up in a variety of forms.
  • Stalagmites rise up from the floor of the caves. In fact, stalagmites form due to dripping water from the surface or through the thin pipe
  • The stalagmite and stalactite eventually fuse to give rise to columns and pillars of different diameters.
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September 2022
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