The word petrology by its definition is the branch of geology which deals with the study of rocks and the conditions under which they were formed. The field of petrology utilizes the classical principles of basic and optical mineralogy and petrography in the study of both the composition and textural properties of the rocks which aids the description of the conditions under which the rocks were formed. The studied area, Ife south, Osun state is located in the transition zone between the crystalline and non crystalline rocks in southwestern Nigeria. It covers and average area of about 12.5km2. the area is found within the Nigerian basement complex, it is underlain by the gneiss-diorite complex whose rock units were found to include; migmatite gneiss, granitic gneiss, diorite, granite and porphyritic granite.
The project was carried to achieve the following objectives;
·        To determine the local geology of the mapped area.
·        To produce a detailed geological map of the area.
·        To enhance the success of these project objectives, fresh samples were collected from the field on available outcrops, measurements of strike and dip values were taken accordingly and thin sections prepared from the different rock types encountered.    
The study area covers a sectioned plot, south of area IV, located in Ife south local government, south of Osun state. Osun state is located in the western part of Nigeria, about 40km from Ile-Ife and 37km from Akure and is a part of the basement complex of southwestern Nigeria. The study area approximately lies between lat 707.5’ and 7010’N and long 4032.5’ and 4032’ E, with an estimated area of 12.5km2. Geological mapping was carried out at Okusu area and its neighboring villages which are; Okusu Aloro, Ago Nathaniel, Odesan, Amulasaliu and Amula odunlade to know the different rock types in the area.
The studied area is located between lat 707.5’ and 7010’N and long 4032.5’ and 4032’ E which lies n the southwestern part of Nigeria. The major villages are Okusualoro, Okusu area, ago Nathaniel, Odesan, Amulasaliu and Amulaodunlade and it covers area of about 12.5km2. Accessibility is poor, most of the routes (mostly footpath) shown on the available topograpghical map has been covered with vegetation due to abandonments of settlement and or farmstead.
The mapped area is characterized by both high and lowland relief which is generally classified as rolling topography characterized by hills, valleys and gently undulating plains with occasional hills. The area has a rugged topography; some hills are high while others are low lying. In general, the local topographic inequality can be accounted for primarily by the differential weathering induced by geological factors such as litholigical differences, the general structural attitude and varying degrees of jointing. The topography of the studied area rises up to about 500 meters and 450 meters above the mean sea level in some places with the highest hill occurring in the northern part of the mapped area. The topography of the area coupled with its climatic condition has imparted greatly in the weathering of the rocks reducing most of the gneisses to clayey material and schist to clayey materials leaving the quartz veins and a lot of pegmatite veins exposed due to the low resistance of gneisses and schist to weathering.    
The studied area is characterized by dendritic drainage pattern because of the ruggedness of the topography and the differential resistance of the lithological units of the geology of the area, it is made up of hills and valleys impacting a dendritic patterm of drainage system . However because of the even distributions of the river system, forming the dendrtic pattern of drainage system and the climatic conditions of the area, the vegetation is ever green with tall trees and shrubs. The mapped area falls within the guinea coca trees and characterized by the presence of tall grasses, tall trees with broad leaves. The tress are scattered and deciduous (they shed their leaves during the dry season). The soil type mostly encountered in the mapped area includes the lateritic sol having a reddish colour due to the presence of iron. The soil is sticky when wet and hard when dry. This soil type impart on the occupation and the type of crops grown in the area which is extensively tubers like yam, cassava, cocoyam and tree crops like cocoa trees and plantain due to the accumulation of nutrients in the B-horizon. The soil type contains iron and aluminium compounds.
The mapped area falls within the tropical hinterland characterized by moderate rainfall averaged between 2000 to 3000mm. it has 4 to 5 months of little rainfall and double maxima of rainfall. It has a mean temperature range of 25 to 270c (about 75 to 800).
The major occupation of the inhabitants of Okusu area and its environ is mainly farming, for both subsistence and commercial purpose. Major crops produced are cash crops like cocoa and oil palm. Most of these farming activities are carried out on the valley and ridges which in most cases have loamy soils. There is widespread occurrence of palm trees and cocoa plantations in the study area through which the production of palm oil and cocoa is possible.