Geological and Environmental Evolution of Sergoit Hill, Uasin Gishu, Kenya

 Author: William Kiptoo et al.

 Abstract 

Sergoit Hill, formerly known as Sergoit Rock, is a residual rock outcrop on the Uasin Gishu Plateau northeast of Eldoret, Kenya. This synthesis integrates regional geological mapping, geochronology literature, and local land use studies to present the phonolitic origin, morphology, and recent ecological trajectory of Sergoit Hill. Evidence supports emplacement of plateau-scale phonolite flows in Miocene times with protruding Basement System gneiss and quartzite inliers producing the residual outcrop. Human land use, grazing, fire, and later protection have driven vegetation change from sparse rock-dominated cover toward denser shrub and tree growth in protected areas. Recommendations for targeted field mapping, geochemical and geochronological sampling, and long-term ecological monitoring are provided.

 1. Introduction

Sergoit Hill is a prominent rock outcrop and local landmark on the Uasin Gishu Plateau northeast of Eldoret. Previous and recent accounts place its elevation near 2,090 meters above sea level and describe a jagged western flank and a smoother eastern slope. Because Sergoit stands above surrounding agricultural plains, it provides a convenient field site to study the interaction between plateau phonolite volcanism, basement inliers, and recent land-use-driven vegetation change.

 2. Geological Background

Plateau-scale studies identify a succession of Miocene phonolite flows across the Uasin Gishu Plateau that originated within rift-related eruptive centers and flowed westwards tens of kilometers. These phonolitic sheets are commonly interbedded with volcaniclastic basal tuff and grits and overlie Basement System crystalline rocks including gneiss and quartzite. Where basement inliers protrude through the phonolite sheets, they form resistant highs analogous to Sergoit Hill. Regional geological maps and Eldoret degree sheet descriptions provide the primary mapping basis.

 3. Morphology and Elevation

Topographic descriptions and local guides report Sergoit near 2,090 meters above sea level with an asymmetric profile—steep on the western side and smoother on the eastern side. This asymmetric morphology is consistent with differential erosion of layered lava flows plus the presence of resistant basement inliers that locally modify flow geometry.

 4. Vegetation History and Land Use

Archival photos and oral histories describe a largely bare rocky outcrop with sparse savanna-like vegetation in the mid-twentieth century. Later twentieth-century settlement, livestock grazing, and recurrent grass fires changed disturbance regimes, encouraging colonization by shrubs and pioneer tree species. Local conservation actions, including tree planting attempts and fencing since the 1990s, have allowed natural regeneration and woody encroachment on protected portions while inhabited adjacent plots remain rocky and sparsely vegetated.

5. Methods and Field Sampling Plan

The fieldwork is designed to map lithologies and contacts, collect representative rock samples for petrography, whole-rock geochemistry, and 40Ar/39Ar dating, and establish ecological plots for vegetation analysis. A 3×3 sampling grid (~250–300 m spacing) is proposed around the approximate coordinates (0.6128° N, 35.3909° E). Samples will include phonolite flow units, gneiss and quartzite inliers, and volcaniclastics. Nine vegetation plots (20×20 m) and six soil pits (50 cm depth) will be established to record species composition, percent cover, and soil characteristics.

 

Figure 1. Schematic location map showing Eldoret and Sergoit Hill sampling grid.

6. Conclusion

Sergoit Hill provides a natural laboratory for studying plateau phonolite volcanism, basement inlier exposure, and coupled human-environment interactions. Vegetation cover has transitioned from sparse rock-dominated savanna to a mixed shrub-tree system under reduced disturbance and fencing. Future research should combine geological mapping, geochemical and geochronological dating, and ecological monitoring to document ongoing landscape evolution.

 

7. References (APA Style)

1      Lippard, S. J. (1973). Plateau phonolite lava flows, Kenya. Geological Magazine.

2      Sanders, L. D. (1963). Geology of the Eldoret Area (Degree Sheet 34 N.W. Quarter). Geological Survey of Kenya.

3      Watson, D., et al. (2025). Tectonostratigraphic evolution of a Miocene half-graben, Kerio Valley Basin, East African Rift. Journal of the Geological Society.

4      Chelule, F. K. (2024). Evaluation of groundwater potential using electrical resistivity: A case of Uasin Gishu plateau. University of Eldoret Thesis.

5      Rotich, B. (2022). Impact of land use and land cover changes on ecosystem services in Eldoret region. Eldoret University Press.

6      Botha, M., et al. (2020). Fire and frost effects at grassland–forest boundaries. Global Ecology and Biogeography.

7      Twendesasa Travel Blog. (2023). Sergoit Hill Wildlife Sanctuary entry and Kruger family conservation history.