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Grassridge rock shelter is located in the high elevation grassland foothills of the Stormberg Mountains in the Eastern Cape of South Africa. This places Grassridge at an important biogeoclimatic intersection between the Drakensberg Mountains, the South African coastal zone, and the interior arid lands of southern Africa. First excavated in 1979, the approximately 1.5 m stratigraphic sequence was divided into two major occupational components: a 50-70 cm thick Later Stone Age component dating between 7-6 ka and an underlying 50-80 cm thick Middle Stone Age component dated to 36 ka at the base. Here we present a reanalysis of the Grassridge stratigraphic sequence that combines new optically stimulated luminescence and radiocarbon age estimates with sedimentological and microbotanical analyses to evaluate site formation processes and the palaeoenvironmental context of human occupations. Results indicate a complex history of geogenic, anthropogenic, and biogenic inputs to the depositional sequence that are interspersed with pulsed human occupation from 43-28 ka, 13.5-11.6 ka, and 7.3-6.8 ka. Microbotanical remains indicate a cooler, drier grassland environment in MIS 3 that transitions to a warmer, moister grassland environment dominated by summer rainfall in the middle of MIS 1. The pulsed occupational sequence identified at Grassridge is characteristic of the Pleistocene and Holocene record across the greater high elevation grassland region of South Africa, which, based on comparison with other currently available evidence, seems linked to a complex system of forager mobility entwined with rapidly fluctuating palaeoenvironments across the last glacial to interglacial transition.

VR003 has been excavated over five seasons between 2009 and 2016. The initial two seasons had the primary objective of assessing the potential of the deposit, and only selected finds were plotted. The final three seasons 2014-2016 involved plotting of all cultural items using different size cut-offs for different classes of material (≥20 mm for lithics, ≥25 mm for bone). Some classes of material, notably OES, were only plotted if modified. All plotted artefacts were assigned unique provenience ID's and individually bagged. Sedimentary aggregates were plotted with a single indicative point location, and volume was quantified as proportional bucket volume (FULL, ¾, ½, ¼). All aggregates were sieved on site through nested 3 mm and 1.5 mm mesh, and the residues from each bagged separately. All plotting was undertaken using a local grid, managed by control points emplaced around the site.We currently recognise nine archaeological horizons and eight geological horizons in the Main Area sequence (1); for the sake of simplicity in this paper we refer to the lower two archaeological horizons (I-08 and I-09) in aggregate as the Lower Deposits. All horizons in the Main Area are associated with the MSA. Inside the small extant shelter (known as Sector III) we identify 23 geological horizons (III-01 through III-23), with Late Holocene LSA comprising the upper 14 strata (0.6-0.9 m depth). In the Main Area, I-04 was assigned to the Howiesons Poort and Still Bay (1), though initially dated by OSL in the Main Area to 45.7 ±2.8 ka. Inside the shelter in Sector III, Howiesons Poort artefacts occur in strata III-18 through III-23, with initial OSL ages of 42.3 ±2.7 ka and 41.7 ±2.9 (1) (Still Bay has yet to be encountered in this part of the site). In both areas these ages are unusually young for the Howiesons Poort. We provide a redating here (SI Geochronology below) using OSL on quartz grains from III-18 to III-20 that places the Howiesons Poort between 71.6±6.2 and 60.8±5.2, consistent with ages elsewhere, with ages for the immediately overlying post-Howiesons Poort stratum III-17 of 55.7 ±4.4 and 66.1 ±5.3 ka (Figure S1).The fossiles directeur for the Howiesons Poort and Still Bay (backed artefacts and bifacial points respectively) occur as a discrete band across the entire site and include a single piece of engraved OES in the Link Trench (Figure S1). In the Main Area there is no recurrence of such artefacts in the underlying deposits, which we assign to the pre-Still Bay MSA.

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Animals often live and forage in groups, though the extent of social organisation varies dramatically among taxa (Ward & Webster, 2016). The key to understanding the form, diversity and maintenance of group living lies in enumerating the balance of benefits and costs across two broad ecological contexts; anti-predator defence and foraging (Krause et al., 2002). In the former case, individuals in