The access road to the park traverses the Clarens Sandstone Formation and Lesotho Basalt Formation contact. Road cutting expose the lithologies and at one point after crossing the contact into the overlying basalt, the route re-exposes the underlying Clarens sandstone at roughly the same elevation, suggesting significant palaeotopography along this geological boundary. This should be investigated further in the context of the high desert dune system characteristic of the Clarens Formation in this area.

The basalt flows are well exposed and display characteristic amygdale form and density zonation and lava flow contacts. Interbedded within the lava flows are thin, lenticular units of sedimentary rock, which accumulated in temporary pools of water on the basalt-flow surface. Interbedded fine-grained sandstone and maroon or greenish grey siltstone or mudrock probably accumulated as wind-blown sediment or as storm runoff flowing across the landscape into ephemeral playa lake depressions. Mud cracks developed on the upper surface of the sediment layers attest to desiccation of these shallow ponds. These depressions probably provided temporary water supplies and supported vegetation, which attracted fauna, hence, the common association of fauna (dinosaur) track ways with the surface of these sedimentary deposits.

The basalt succession is intruded by numerous dolerite dykes and sills, which were conduits to magma feeding high-level fissures from which younger lava flows erupted on the accretionary continental flood basalt surface. The dykes have a dominant NNW-SSE orientation, controlled by basement structure, although intersecting NNE-SSW dykes and fractures are also evident in the Tsehlanyane valley. The homogenous dolerite dykes and sills are similar geochemical composition to the basaltic lava flows but are coarser grained due to slower crystallization at hypabyssal depths below the surface. Some of the dykes intrude the highest basalt flows suggesting that they were feeders to the highest-lying flows, which have been removed by erosion. The different weathering patterns of the dykes and sills is due to their textural homogeneity and the lack of vesicles and amygdales which were formed by concentration of volatile phases in the extrusive phases.

A thick dolerite sill was exploited for construction aggregate in a quarry on the northwestern slopes of the Ts’ehlanyane River valley. The dolerite texture shows relatively homogenous crystal size and shape, without the chilled, glassy groundmass of the basalt flows. Crystal intergrowths between the light-colored, calcic plagioclase feldspar and the Ca, Mg, Fe-rich pyroxenes are characteristic of dolerites in this region. The enhanced resistance to weathering of the dolerite sill result in steeper slopes along its outcrop in the lower valley side slopes.

An important aspect of the dolerite dykes is the preferential weathering, which occurs along the brecciated margins and exploitation of these weathered zones by stream incision. The structural control of dykes on tributary stream valley is a distinctive aspect of the deeply incised river valleys in the region. This geomorphological phenomenon is particularly well developed in two parts of the park. In the Selepeng stream valley, the south-flowing Holomo tributary has exploited a NNE-SSW trending dykes or fracture-related zone of weakness and a similar linear trend is reflected by a tributary on the opposing, southern side of the valley. The influence of this lineament can also be recognized in the orientation of tributary streams across the interfluves in the stream basin to the south. Towards the north, in the northernmost tributaries of the Tsehlanyane River (+3 196 300X, +52 000Y), a crosscutting dyke or fracture pattern has been exploited by tributary incision. Numerous other sites display prominent, aligned tributary orientation across high interfluves.

The 1:250 000 geological map of Lesotho (Lesotho Government, 1982) shows a kimberlite dyke intruding the basalt flow succession along the northern slopes of the Holomo stream valley. This occurrence may well be exposed in the track leading up the Holomo pass and is aligned with the dyke swarm, which is mined for diamonds at Kao Mine. Kimberlite is a highly variable rocktype, a hybrid of various rock fragments derived from the mantle and crust, a variety of megacrysts and variable groundmass (Tankard et al., 1982). It is an alkaline, ultrabasic igneous (hundreds of kilometers) in the upper mantle along fissures or vertical pipes.

There is little evidence of preservation of significant, young surficial deposits on slopes in the park and mass movement through landscape is efficient in removing weathered material. No distinct terraces are preserved along the Tsehlanyane River course at this elevation, suggesting continual incision of the river course.

Despite the relatively steep slopes, well-developed soil profiles have formed within the mafic basalt/dolerite bedrock. Thick, reddish brown, apedal soils are characteristic of well-drained position of relatively warm, north-facing slopes.

The basalt layers are underline by various sedimentary rocks of the Karoo Sequence. However, as the park lies on the upper beds of the lava flows, the entire park is underlain by basalt, which dolerite, give rise to more fertile soils than from sedimentary rocks.

Klug et al (1991) comment on the particular features of the basalt which have influenced soil properties in the Maloti Mountains, and which are pertinent to the soils of the BNR. These are:

The Drakensberg basalt is unusually rich in calcium (in the region of 10.5% CaO;
The lavas contain a high proportion of volcanic glass; and
The textural and hardness variations present in the various basalt layers, which result in steeped profiles which are prominent on many hill slopes, and which emphasize the underlying stratification.

GEOMORPHOLOGY AND TOPOGRAPHY

Description

The entire Maloti Mountain range is extremely rugged, with numerous deeply incised valleys, which have cut into the original surface of the basalt massif. It is considered that a number of episode of uplift and monoclinal warping have occurred, the most recent of which is considered to have taken place at the end of the Pliocene, around 2 million years ago. These involved upward displacement in the region of 900m (Partridge & Maud, 1987), and have led to accelerated rejuvenation of the river system, with associated dissection of very deep valleys. Hill slopes are typically steep to very steep, with marked valley-head asymmetry. The major drainage lines trend east west, with south-facing slopes markedly steeper than warmer aspect north-facing slopes.

Four ecological zones are commonly recognized in tropical mountain ecology. In order of increasing altitude, these are the Tropical, Alpine and Nival zones. The Tropical and Nival (permanent snow cover) zones are absent from the Mountain province of Lesotho

The park is comprised principally of the Subalpine Belt, but includes a limited area of Alpine Tundra, up to 3 112 m asl. Its lowest point (c. 1940 m asl) is just above the interface between the Subalpine and Montane Belts.

High Plateau

This consists of rolling upland plateau, which extends westwards of the Drakensberg escarpment. The system is everywhere bounded by a sharp convex break of slope and is entirely underline by basaltic rocks. The terrain consist if hilly and steeped land, interspersed with a few rolling valleys.
The hilltops are flat with concordant summits; the peaks rise to over 3 300m and the valley floors are at 2 900-3050 m.

Mountain summits present in the High Plateau land system are considered to represent the lowered remnants of the original basalt surface. The highest peaks of the mountains stand above what appears to be a continuous erosion surface. Remnants of this may be seen in the upper reaches of the Park.

High Mountain Flats

The High Mountains Flats land system includes flat-floored valleys, separated by steep slopes, rising to the High Plateau. At about 2 900 m, they are cut at a lower level than the surrounding plateau, and have been ascribe to the post-Gondwana erosion surface.

The highest reaches of the Tsehlanyane valley system lie in a “High Mountains Flats” land system.

Higher Slopes

These are steep valley slopes descending from the High Plateau that merge into topographically similar slopes below. The system largely consist of straight simple basaltic slopes, falling from about 3 200 m to 2 600-2 700 m into the Subalpine Belt.

In the deeply incised valley system of the park, current dynamic geomorphological activities such as valley-head erosion are currently taking place. The steeped profiles of some of the hill-slopes within the park, indications of the underlying differences in composition of the basalt layers, may be observed.

Soils and substrates

Description

As indicated above, the entire area is underline by volcanic basalt, through which dolerite dykes and sills have intruded. Extremely active geological erosion has produced highly dissected country of high relief, with mass wastage processes predominating. Indications of this are that analysis of the terrain of the Maloti Mountains indicated that 96.3% has slopes of from 12-100% and only 3.6% has slopes of less than 12% (Klug et al., 1991). The park itself consists principally of the cliff faces, and precipitous scarp and valley slopes of the Subalpine Belt. The mass wastage processes are extremely active, and few areas of deep soil are present in the park. A high proportion of the park in fact consists of thin skeletal soils lying directly over rock. In such a situation, it may be expected that the soils are relatively young and shallow, derived from the underlying basalt or dolerites.

Soils of the summit areas and valley sides are general shallow (< 600mm) and of medium texture (loams to clay loams). The soils are dark in color, due to very high levels of organic matter (in the range of from 6-16%). They are well aggregated, porous, base-rich (due to the calcium-rich parent material), and have near-neutral pH values. Calcareous nodules are visible in some profiles. Freeze-thaw processes, particularly at higher elevations and on south-facing slopes, contribute to the excellence state of granulation and uniformity of profile. The only regional difference of note is the reddish colouration, due to higher rates of weathering, found in soils of north-facing slopes.

The two most common soils of the plateau areas of the Alpine Belt (above 2 750 m) are the Popa-Rockland (basalt)-Matsana Association, and the Matsana-Fusi-Popa Association (Soil Survey Conservation Division 1979, Schmitz & Rooyani 1987, Consult 4 1996). The homogeneity of the parent material results in soils, which differ by degree in texture, depth and drainage capacity, rather than by structure.

Hydromorphic (Phecela) soils are found in the minor valley floors and in other depressed sites. These are deep (>1 m), have a fluctuating water table, higher organic matter contents than soils of the valley sides and summit, neutral pH values and high base status, which might not otherwise be expected in such a highly-leached environment.

Then Alpine and Subalpine Belts are sensitive environments, which respond rapidly to disturbances and poor land-use methods. In the low ambient temperatures, which prevail, soil genesis rates are extremely slow. While the soils are inherently stable by virtue of their high organic matter content and favourable state of aggregation, they are nevertheless susceptible to erosion from high intensity rain storm, particularly if the vegetation cover on steep slopes is decreased. Erosion gullies form rapidly following poor siting of roads, paths or other forms of development. Recovery rates are extremely slow. Bared or eroded areas may not recover in many decades.