Landforms Evolution of Wadi Qudaid Area, West Central Arabian Shield, Saudi Arabia: An Example of the Role of the Geological Factors in the Urban Extensions

Wadi Qudaid is located in the west central part of Saudi Arabia. It about 135 km to the northeast of Jeddah city along Al Haramein highway and it represents the upstream of the very large alluvial plain along the Red Sea coast. It runs in NE direction parallel to many wadis of the west central part of Saudi Arabia i.e. Wadi Fatima, wadi Sitarah. The wadi floor is filled by Quaternary deposits which represent good groundwater aquifer. Geologically, the present-day residual landforms of Wadi Qudaid are composed mainly of Precambrian Arabia shield rocks overlained by Tertiary sedimentary rocks and finally harrat (Tertiary volcanic). The Precambrian rocks are represented by 1) a lower layered basic and intermediate volcanic and the intercalated volcaniclastics. This unit is correlatable with Samran Group, 2) an upper layered acidic volcanic and the intercalated acidic volcaniclastics. The Arabian shield rocks are intensively folded and dragged along the major NE faults. They are directly overlain by Tertiary basic volcanic (harrat) and the related volcanoclastic red beds). The main geomorphologic elements of the study area include plateau, scarps, and the wadi floor. The plateau is represented by the black basaltic sheet that contains some semi-rounded depressions the wadies sides by scarp retreat. The peniplanation stages of the erosion cycles are reached in the southwestern and the central part of the wadi where fast peniplained areas were formed. The results of this study revealed the role of the different geological processes (lithology, structural elements and climatic conditions) in the distribution of present-day human populations in urban extensions. Qudaid, Dhubaya-Jumah, Dabyah, Al Khamrah, Almansa and Al Mas-samah are the main villages of Wadi Qudaid area.


Literature Review
Wadi Qudaid is occupied by many villages and contains a lot of green private farms. The present-day human population of Wadi Qudaid area depends on the private and governmental wells. The area is of good climate in the winter, but it is very hot in the summer periods. The geomorphology, hydrology and drainage morphometry of Jeddah-Makkah district appears in the publications [1]- [8]. The climate of Jeddah Governate is a hot arid desert type, with some rainfalls in the spring season; the temperature reaches a maximum of 49˚C within the period between March and August, and a minimum of 17.3˚C during the period between January and February. The groundwater of Wadi Qudaid area is suitable for agriculture purposes and the results of the chemical analyses show the normal content of Ca, Mg, Na, Cl, SO 4 , and HCO 3 [9]. They also concluded that the groundwater aquifer of the northeastern part has shallower depths than in the southern part and the Quaternary deposits are thinner in the northeastern part than the southern part (downstream). The study also revealed that the groundwater is of good quality in the upstream (NE) area than the downstream (SW).

Aims of the Study
This present study aims to give a detailed description of the different landforms of Wadi Qudaid area. Also, the study deals with the clarification of the different geomorphologic processes that resulted in the formation and evolution of the landforms of the study area. The second aim of this study is to clarify the role of the different geological factors in the human populations and urban extensions in the study area.

Methods of the Study
This study is based on detailed field measurements and descriptions of the different landforms. Global mapper and Arc GIS programs are used in the preparation of the satellite images and geologic maps as well as the topographic and

Geologic Setting
The study area is of characteristic geologic and geomorphologic setting. It represents a part of the Arabia shield rocks of the west central part of Saudi Arabia ( Figure 1). It contains many NE trending wadies i.e. wadi Fatima, Wadi Qudaid, and wadi Sitarah. Wadi Ashamiyah and wadi Ashumaysi are NW trending ( Figure 1). The geology of Makkah area is described by [10] to include older basement rocks which overlay, by the sedimentary rocks, then the volcanic rocks  This unit is composed of interbedded rhyolite, rhyolitic tuffs, chert, and quartz bands ( Figure 4(C)). Thin black basic bands are present within the middle and upper part of the succession ( Figure 5(A)). White color carbonates and/or quartz bands are also present ( Figure 5(A)). The Tertiary sedimentary succession is composed of volcaniclastic red beds, sandstones, siltstones, basalts, and carbonates ( Figure 5(B)). This unit is of characteristic color and it is excavated

Structural Elements of Wadi Qudaid
The structural elements of Makkah district have previously been studied by many authors. [12] studied the area between Jeddah and Al Taif and concluded that the area is subdivided in terms of geology into two units. These are the Precambrian basement, which is built up by intrusive series and layered rocks and the Tertiary to Quaternary basaltic effusives. They also concluded that the region has been affected by various periods which, on one hand, have to be addressed to Precambrian orogeneses and on the other, to the Tertiary-Quaternary motions in connection with the Red Sea rifting. They also pointed out that, the Satellite Image Map (SIM) shows ENE-trending fractures of Jeddah/Taif area as "first order lineaments" of Precambrian origin ( Figure 6). According to this study, the SIM interpreted NNW Red Sea Rift Fractures as first order lineaments and the bending of the NNW trend is clearly visible on this map. Regional considerations suggest that NNE fractures of the area belong to the conjugate set Tertiary fractures.
The study area is dominated by NE lineaments [12] ( Figure 6). The structural elements of the study area into faults that are related to the red sea tectonics other three main types of fault sets affecting the Precambrian crystalline rocks [10] [13]. These sets have N-NW, NE, and E to NE trends. He recommends the avoidance of these fault systems in locating the potential sites of radioactive disposal sites.

Geomorphology of Wadi Qudaid
On the satellite images, the study area represents an ideal example for the etched or excavated areas. W. Qudaid is recharged from many tributaries from the eastern and northeastern parts of the study area (Figure 7(A)). Within its course, it runs in E-W direction, especially in the lower part and in NE direction in the upper part (upstream). The present-day landforms are either represented by Arabian shield rocks, Tertiary sedimentary succession, Tertiary basalts (harrat), and finally Quaternary deposits (Figure 7(B)). Figure 6. Lineament map of Wadi Qudaid area [12].

Geomorphology of Southern Sector
This part of Wadi Qudaid represents the downstream of the wadi. It is highly denudated and sculptured where the residual land forms percentage do not exceed 40% of the total volume of rocks ( Figure 8(A), Figure 8(B)). The relicts landforms take different forms and sizes according to the degree of weathering and denudation. The shapes and sizes of these landforms depend mainly upon the intensity of weathering and sculpturing processes, which is variable from one area to another one. These processes depend on the variation in topography, rock types, and intensity of structural elements. All the landforms are nearly having the same elevation and also the same slope elements. The progress of weathering processes and evaluation of the different landforms is shown below as follow: 1) The different geomorphologic units in the study area 2) The Flat Plateau Which form the main geomorphologic unit where it is present as flat-topped areas of Arabian shield rocks overlain by the black Tertiary basaltic rocks (harrat) (Figure 9(A)). Some large areas of the Arabian shield rocks are not overlained by harrat and these domains are not flat-topped. The progressive desiccation of this plateau (Figure 9(B)) led to the formation of v-shaped areas surrounded by low-lying cone hills (Figure 9(B)).

3) Cone hills and depressions
The progress of the weathering processes led to the development of small de-  (straight) slope, and finally the pediment (lower concavity). The presence of these four slope elements reflects the formats of these landforms by progressive and subsequent stages of weathering.

5) Sculpturing and denudation
The progress of these processes led to the landform evaluation of the study area. The structural elements have a very clear role in the evolution of the landforms where they interplay with the different folds and faults (Figure 11(B)) which led to the exposing of different rock units.

Geomorphology of the Central and Northern Sectors
In this part, the wadies become narrow and the interfluves become wider. The total volume of the residual landforms is very high. The interfluves are very narrow and very deep ( Figure 12(A), Figure 12(B)). The recharge area is located in the east and northeast. The elevation of wadi floors is slightly higher than those  The contour maps of this part show the low topography of wide areas that are suitable for the human population and urban extension (Figure 13(B)). The progressive of this form of weathering led to the evolution of landforms by the scarp retreated and lowering of elevation as a result of the complete erosion of the hard-resistant basaltic cap (Figure 13(B)). This will be followed by peneplanation processes and complete removal of the residual landforms and formation of wide peneplaned areas of low topography and suitable for the human population (Figure 13(B)). These areas are now filled by the Quaternary deposits which represent groundwater aquifer and represent the suitable cultivated lands of Wadi Qudaid area.
-Scarps of the Plateau These scarps surround the wide interfluves of the main course of W. Qudaid. In some areas of the northeastern part of Wadi Qudaid (Figure 14(A)), there is a wide area suitable for the urban extensions on both sides of the wadi (Fig  14(A)). These areas are progressively denudated by the formation of very narrow wadies along main faults and fractures within the scarps (Plateau, Figure  14(B)). The progress of sculpturing processes by the side retreat of the scarps of the interfluves led to the formation of wide areas suitable for the human population ( Figure 14(C), Figure 15(A)).
These interfluves areas are of rounded and symmetric slope. The slope analyses of these landforms revealed these processes of one rectilinear slope unit with a high angle in its lower part. In highly denudated areas, the lower part of the rectilinear slope becomes eroded giving rise to lower concave slope (pediment, Figure 15(B), Figure 15(C)) which is associated with the formation of some promontories in the scarps. In some landforms, the rectilinear slope is present either on the two units of the Arabian shield rocks (Figure 16

Landforms Evolution
The term landforms evolution has been used by many authors to describe the changing and evolution of the geomorphologic phenomenon. The landscapes are dynamic, acutely sensitive to natural and artificial perturbation and they can   Figure 19(Ib2) and Figure 19(Ic1), and Figure 19(Ic2)).
The wide interfluves were formed as a result of the continuous slope retreat  destruction of this basaltic sheet after its formation [16]. At this stage, the wadies become wider and the interfluves become narrower (Figure 18(IIA), Figure  18(IIB)). On the satellite images, this stage of landforms evolution may lead to the destruction and removal of the topmost basalts which led to the exposing of the underlying softer units (Figure 20    The bore hole data of hydrogeology and hydrochemistry study published by [9]. 3) The quarrying processes for concrete, gravel within the course of Wadi Qudaid led the change and the natural topography of the wadi floor.
4) The haphazard drilling of water wells which resulted in the lowering of the groundwater level of the study area.

Discussions and Conclusions
Deserts on old creations, occurring in the Indian subcontinent, Africa, Arabia and Australia, have less relief than the Atacama, but are not necessarily a dune-covered land with low relief. The landform on such a geological framework is produced by processes of erosion and deposition carried out by both water and wind. The surface material includes bare rock, gravel, sand and riverine silt. According to [17] the rainfall in arid regions rainfall is localized, intense and erratic over both time and space. Widespread rain occurs only when unusual meteorological incidents occur.
Three main concepts are previously postulated for the describing the mechanism of erosion processes and formation of depressions, wadies, lateritic products and karst landforms and the formation of the lower erosion surface from the higher older surface is described as etchplanation [18], and the related processes  [20]. According to [16], from humid to present day and semi-arid. The variation in the groundwater quality was studied by [23]. They concluded that the groundwater of the upstream part is of better quality than the downstream part.
Although the processes of erosion and landforms evolution led to the formation of wide peneplain areas suitable for the human population and urban extension, there is many highways are constructed in the middle part of the wadi.