◙ Objectives of the lecture: ●Identification of the symmetrical and asymmetrical profile of the valleys and understanding the reasons of the asymmetrical cross profile. ●Identification of different valleys according to the stage of valley, depending on the relationship between the direction of stream flow and dip direction of rock strata, also classification depending on the type of geologic structure which have controlled their development. ●Identification of different drainage patterns and understanding effects of lithology and structure in the development of each type. ◙ Lecture contents: ●Base level. ●Graded stream. ●Cross profile of valleys : -Symmetrical cross profile. -Asymmetrical cross profile. ●Classification of valleys : -Classification according to the geomorphic cycle stage. -Genetic classification (according to their origin) -Structural control classification. -Classification according to the effects of change of base level. ●Drainage patterns and their significances.
Base level: The base level of a river or stream is the lowest point to which it can flow, often referred to as the 'mouth' of the river. For large rivers, sea level is usually the base level, but a large river or lake is likewise the base level for tributary streams.sea levellake All rivers and streams erode toward sea level, which is also known as the "ultimate base level. A low base level is a prerequisite for the formation of karst topography, a network of sinkholes and caverns that can develop as slightly acidic groundwater enlarges joints (by solution) in limestone rock.karst topographysinkholescavernssolutionlimestone Base level is also significant for subsurface drainage.
Base level: A downward limit to the valley deepening, or it is a maximum depth which can be reached by a valley. According to Davis the base level is two main types: 1- general (permanent) base level. 2- local base level. Graded stream: A stream in which, over a period of years, slope is adjusted to yield the velocity required for transportation of the load supplied from the drainage basin. A stream that has a long profile that is in equilibrium with the general slope of the landscape. Stream's maintain their grade through a balance between erosion, transportation, and deposition. Erosion removes material from bumps in the profile and deposition fills in dips. A graded profile is concave and smooth.
1)A stream early in the geomorphic cycle attains that slope of gradient which under existing conditions of discharge and channel characteristics is just able (sufficient) for transporting of its load. 2)A graded stream has a profile of equilibrium (smoothly concave upward curve) which decreased in slope gradually and systematically down valley. 3)Some streams may have both graded and ungraded stretches along their courses and abrupt changes in gradient is seen and it is marked by rapid or waterfalls, the stream profile is called interrupted profile and the sharp inflection area is known as Kinkpoint. Higher above base level, water has more potential energy, which gets converted to kinetic energy as it flows downslope.
The longitudinal profile of a valley is the gradient throughout its length. The graded stream is a stream in equilibrium
-A valley is a low area between hills, often with a river running through it. Cross- section (cross profile) of valleys In geology, a valley or dale is a depression that is longer than it is wide.geologydaledepression -The terms U-shaped and V-shaped are descriptive terms of vgeography to characterize the form of valleys.U-shapedgeography -A valley formed by flowing water, or river valley, is usually V-shaped. The exact shape will depend on the characteristics of the stream flowing through it. Rivers with steep gradients, as in mountain ranges, produce steep walls and a narrow bottom. -Those valleys produced by tectonic action are called rift valleys. Very narrow, deep valleys of similar appearance are called gorges. Both of these latter types are commonly cut in flat-lying strata but may occur in other geological situations. -Valleys formed by river action typically have a concave upward profile, steep in the headwaters and gentle in the lower reaches. The lower end of such a profile is adjusted to an effective lower limit of erosion defined by the baselevel.
The cross section of a valley sheds light on the geomorphic history and influence of the local geologic and climatic controls on the valley. The cross-section may be symmetrical in which both valley slope sides are equal (V, U), and the second type is asymmetrical in which both valley slope sides are not equal. The contact of the river deposits with the eroded bedrock surface below them is called the strath.
If the river experiences a period of incision, these deposits can be left stranded above the river’s new level, forming a strath terrace. The top of the terrace is now called a tread.
Although slopes and rivers are often studied separately by process geomorphologists, hills and valleys are the features that dominate landscapes. The cross profiles of valleys involve a combination of fluvial and hill slope processes. In upland areas cross profiles of valleys are often narrow and deep. Canyon morphologies are most common. Further downstream, valley floors are wider and often dominated by floodplains and terraces.
A- Production of the under-cut and slip-off slopes due to the effect of stream meandering and lateral erosion so an under-cut over steepening slope of one of the valley side and a slip-off slope of the other valley side are formed. The asymmetrical of the cross section may due to:
B- Structural controls: the structural benches are formed due to alternating of strong and weak rocks at nearly horizontal attitude and these benches are not continuous for long distances and may be present at one side of the valley and not on the other side which causing asymmetry in the cross section. C- Homoclinal shifting: when a valley roughly parallel to the strike of the inclined beds, the stream tries to shift its position downward or down the dip of the beds (even following the surface of resistant beds) and this shifting causes an asymmetry in the cross section. D- Direct or indirect effects of microclimatic differences on the two sides of the valley.
Classification of valleys A. Classification according to the stages (their characteristics) in the geomorphic cycle, valleys are classified into: 1) Young valley. 2) Mature valley. 3) Old valley. Because of the great diversity of morphological features among rivers, a stream classification system was developed to stratify and describe various river types.
B. Genetic classification of valley (according to their origin), There are five generic classifications: 1) Consequent valleys: - are streams whose course is a direct consequence of the original slope of the surface upon which it developed, i.e., -streams that follow slope of the land over which they originally formed.
2) Subsequent valleys: generally follow paths determined by the weak rock belts. b-Their courses follows the strike of the beds, so sometimes they are called strike or longitudinal valleys. a-They shifted from the origional consequent ones to belts of erosion (weak) rocks.
3) Insequent valleys: They show no apparent adjustment to structural and lithiological controls. 4) Obsequent valleys: They drain in direction opposite to the original consequent valley. 5) Resequent valleys: They flow in the same direction as the original consequent valleys but at a lower topographic level. These streams develop later and are generally a tributary to a subsequent stream. streams have an almost random drainage often forming dendritic patterns. These are typically tributaries and have developed by a headward erosion on a horizontally stratified belt or on homogeneous rocks.
C- Structural control classification Depending on the type of the geologic structures which have controlled their development, valleys are classified into the: 1- Homoclinal valleys: They follow weaker rocks along the flanks (limbs) of folds and homoclinal structures where alternating weak and strong strata having moderate to high dips. 2- Anticlinal valleys: They follow the axes of breached anticlines 3- Synclinal valleys: They follow the axes of breached syncline
4- Fault valleys: The streams follow depressions consequent upon faulting. 5- Fault line valleys: They are subsequent valleys following a fault line. 6- Joint valley : They are minor valley or sections of valleys controlled by major joint systems. 7- Transverse valleys : They are valleys whose courses cut across the geological structures.
D-Classification according to effects of change of base level Many valleys show the effects of change of base level due to either diastropic or eustatic rise or fall of sea level. Two types of valleys are recognized, they are: 1- Drowned valleys: They develop from rise of sea level. 2- Rejuvenated valley: They develop form lowering of sea level.
Drainage pattern and their significance Drainage system is the pattern formed by the streams, rivers, and lakes in a particular drainage basin.streamsriverslakesdrainage basin The number, size, and shape of the drainage basins found in an area varies and the larger the topographic map, the more information on the drainage basin is available.topographic map Drainage Basin is a region contains the primary and secondary tributaries and separated from another drainage basin by water divide line Drainage Pattern is the particular design (plain)which the individual stream segment courses collectively form.
Drainage patterns reflect the influence of the following factors: 1-Initial slopes. 2-Inequality in rock hardness. 3-Structural control. 4-Recent diastrophism. 5-Recent geologic and geomorphic history of the drainage basin. So the study of drainage patterns is extremely helpful in the interpretation of geomorphic features and represents one of the more practical approaches to understanding of the structural and lithologic control of landform evolution. Note: It can be refer to the patterns of individual stream courses in contrast to the regional pattern, by “Channel pattern”. Channel pattern: It is the configuration of the stream channel as viewed from the air. ⋆ Many descriptive terms may be applied to channel pattern, they can be reduced to three(3) patterns: 1- Straight channel pattern. 2- Braided channel pattern. 3- Meandering channel pattern.
◙ References: 1-Thornbury, W. D., 1969. Principles of geomorphology (2 nd ed.). John Wiley & Sons, Inc., New York, 594p. 2-Bloom, A. L., 2002. Geomorphology- A systematic analysis of Late Cenozoic landforms (3 rd ed.). Prentice-Hall of India Private Limited, New Delhi, 482p. 3-Ritter, D. F., 1986. Process geomorphology (2 nd ed.). Wm. C. Brown Publishers, lowa, USA, 579p. www. http://en.wikipedia.org/wiki/Drainage_system.