## 3 Results
### Lake Chilwa Paleo-Environment Control
#### 3.1.2 Evolution of Lake Chilwa  
Geological and biological fauna evidence suggest that Lake Chilwa was connected to Lake Chiuta (Figure 1), the Lugenda River and ultimately the Indian Ocean until the late Pleistocene [48]. Prior, ancestral Lake Chilwa is proposed to have arisen from ponding of the ancestral Phalombe-Lugenda River, which was increasingly unable to maintain its approximate south-to-north course across the basin due to later Tertiary and Quaternary uplifting of the Shire Highlands and down-warping of the central Chilwa-Phalombe Plain during and post Rift Valley formation [5]. Evidence of a deeply-cut river system crossing the basin is provided by sediment depths of 40 to 80 m along the basin axis and close to the major influent rivers, including the Phalombe. Deeper sediments logged in boreholesappear increasingly fluvial rather than lacustrine. [PERSON] [49] reports 'upwards fining sequences of gravels, sands and clays typical of braided rivers' in boreholes to the north of the Phalombe River [5]. Figure 3: Estimated former Lake Chilwa extent based on relict shoreline elevations contoured over current ground surface shown against a backdrop of High Level and Lake Terrace deposits (locations of these deposits are based on Garson and Walshaw [31]) for: (**a**) Lake Chilwa Basin area; (**b**) close-up of Phalombe District and surroundings. Data are shown over a satellite image backdrop. The oldest high-level lacustrine deposits (Figure 3b) are encountered up to 60 m above current lake levels around Zomba, and perhaps to 100 m around Mulanje. Hills present in the Chilwa-Phalombe Plain are assumed islands under those former lake conditions [31]. Current variation in heights of remnant patches of high-level alluvium in the Chilwa-Chiuta area suggest deposited material in the then open lake (_c._ 130 km long by 65 km wide) was gently tilted to the NNE followed by considerable depression of the central tilted platform. Limits of the high-level lacustrine deposits are typically uncertain being often obscured by later coverings of sandy colluvium from adjoining mountains. They comprise variable thicknesses of black-dark grey and yellow-grey clays muds and silts. Boreholes may encounter thin layers of gravel and fine sand aquifer resource below these low-permeability deposits [31]. Relict shorelines allow discernment of Lake Chilwa closure. The 652 m single lake comprising both Chilwa and Chiuta drained to the Indian Ocean and was succeeded by a 637 m lake similarly draining [5]. A projected slow fall in the lake level from 637 m to 631 m was accompanied by marked changes in the sedimentary environment. The 631 m shoreline is associated with extensive sandy beaches, ridges, and bars backed by low sand dunes [48]. These rise in height north-eastwards to merge with the prominent arch-shaped sand bar which now separates Lake Chilwa from Lake Chiuta, the Chilwa-Chiuta sandbar, that prevented outlet and drainage to the Indian Ocean. Fluvial sediments entering the western side of lake Chilwa, especially from the Sumulu and Domasi Rivers, form the lower terrace deposits of coarse to medium sands. The 631 m lake represents the onset of closed lake conditions and is the largest endhoric lake possible. The 627 m lake shorelines comprise a series of minor shorelines represented by small sandy beaches locally cutting into the deposits laid down at the 631 m lake stage. The 627 m shoreline represents the boundary of the ecosystem of the lake, swamp (wetland) and floodplain [5]. Lake Chilwa closure corresponding to the 631 m lake contour outline occurred in relatively recent times, probably within the last 15,000 years (15 ka) [47; 50]. Sediment dating of the Chilwa-Chiuta sandbar with depth indicates a range of ages for lake high-stand phases of 44-33, 26-21 and 18-12 ka [7]. A closed lake level maximum of 631 m though was still perhaps attainable within the last few centuries [7], possibly as late as the 18 th century [42].