Table 1.
List of geographical terms, including rock type, with a short description and characteristics
| Name | Short description | Characteristics |
|---|---|---|
| M1 | Crystalline, reef and detrital limestone | Hard and dense crystalline Miocene limestone consisting of broken limestone, crushed coal, shell fragments and bands of flint. Sandy and gritty, formed as discontinuous reef, cavernous in places. Supplies water to lakes at Bambi and many springs and well, including cave wells. |
| M3 | Marls, sandy clays and clayey sands | Forms the main base rock of Unguja. Bluish grey to bluish green in colour comprising of dense, roughly sorted Miocene chalky rocks with veins of gravel which weather to a red, yellow, or brown colour. |
| Q1 | Soils, laterites, alluvial and colluvial deposits | Mixture of red, brown and black Quaternary soils rich in iron oxide typical of tropical environments. This fine grained soil maintains a water table forming an underground aquifer which provides a source of water for hillside springs. |
| Q2 | Coralline and reef limestone | White, cream or yellow-brown Quaternary limestone which tends to be grey along rocky and jagged outcrops. Notably free from iron staining. Common across Unguja, except the north-eastern region, forming the island’s main underground aquifer. Frequently cavernous forming many cave wells in conjunction with M1. |
| Q3 | Marine and fluvial sands and sandstone | Sands mixed with shell fragments, fish bones and sharks’ teeth which are lightly cemented forming grey, coarse Quaternary sandstone. Provides water for pumped wells at Kisima Mchanga and Cheju. |
| Doline | Bowl-shaped depression | Bowl-shaped closed depressions (1–1,000 m in diameter) formed by the dissolution of limestone rocks by corrosive groundwater (carbonic acid from the reaction of water with calcium carbonate which is abundant in limestone rocks). Fine-grained soils often drain into these features. |
| Infiltration | The rate at which a soil or rock is able to absorb water | Low infiltrating soils on Unguja are relatively fine grained, well-weathered soils typical of the Q1 geology type. Rainwater and irrigation will absorb relatively slowly into the soil helping to keep soils saturated and retain water at the surface. Conversely, the Q2 rock type is characterised by high infiltration due to cracks and crevasses. |
| Regolith | Fine-grained weathered material | Loose, fine-grained material formed by weathering of rocks. |
| Terra rossa | Red clay soil | Red clay soil produced by the weathering of limestone. |
| Perennial stream | A river channel that runs continuously throughout the year |
See Figure 3 for a map of geology types.