Deep time in an afternoon

You are walking across a bare, gently sloping rock floor at the coast. The surface is smoothed and polished, scored with long parallel scratches all running the same way, and here and there an out-of-place boulder of different rock sits stranded on top, as if dropped from nowhere. Where the floor's edge is cut away, the rock beneath is not flat-lying — it stands tilted, almost on end. What are you standing on?

Cues: A bare rock pavement, smoothed and polished, not sand or soil · Long parallel scratches and grooves all running one direction · An isolated 'erratic' boulder of foreign rock stranded on the surface · The rock beneath the polished floor stands tilted, almost on end, rather than lying flat

An ice-scoured Permian pavement lying directly across the tilted, folded rift-basin sediment of the Adelaide Rift ComplexThe scoured floor of the older 'Snowball Earth' Marinoan glaciation, at the very place Mawson and Sprigg named itA wave-cut rock platform, polished and grooved by surf and moving sandAn old stone quarry floor, its parallel grooves cut by machinery

The polish, parallel scratches and stranded erratic are the signature of a moving ice sheet, not surf or machinery — surf rounds and sorts, it does not leave one-directional scratches, and a quarry floor would not carry a foreign boulder (Bourman et al. 2016). The tilted rock the ice ground across is the older tell: it is the folded, upended Adelaide Rift Complex, crushed into a mountain range by the Delamerian Orogeny long before this ice ever arrived. This pavement's ice is younger still than the Sturtian and Marinoan 'Snowball Earth' glaciations named a few kilometres up the coast at Marino — a different, far older freeze entirely, so the two must not be conflated (Mawson & Sprigg 1950).

You are driving into Adelaide along the eastern edge of the plain. Ahead the ranges do not roll upward gradually the way a long-eroded old landscape should — instead they stand as an abrupt, blue-grey wall, climbing some two hundred metres in a single uninterrupted step. Paddocks and remnant woodland run right up the foot of that wall; no suburb climbs its face. What are you looking at, and why does it stand up like that?

Cues: An abrupt, blue-grey wall rising ~200 m in one uninterrupted step, not a long worn-down roll · Paddocks and remnant woodland running right up the foot of the range, with no housing climbing the face itself · The step follows a straight, fault-controlled line, not an irregular erosional edge

The Eden-Burnside Fault scarp — old basement rock thrust up and over the young plains, its face kept clear of housing by the Hills Face Zone planning overlayAn ordinary eroded hill front, worn down uniformly like the rest of an old, quiet continentA sea cliff cut by wave action when the gulf last stood higher against the rangesA firebreak cleared deliberately along the range front as a seismic safety buffer

The abrupt, wall-like step is the signature of active reverse faulting, not ordinary erosion: along the Eden-Burnside Fault, ancient Proterozoic basement is being thrust west and up over the young alluvium of the plains at a rate averaged around fifty metres per million years over the last five million years (Sandiford 2003; Quigley, Clark & Sandiford 2010). The clear, paddock-and-woodland face is not a seismic buffer someone designed — it is the Hills Face Zone, a planning overlay dating from the 1960s that has kept the range front free of housing for reasons of view and backdrop, leaving the region's most active geological feature looking, by accident, like parkland.

You're standing at the railed lookout on top of Mount Lofty on a clear day. Facing west, a bright thread of gulf water lies beyond a grey-green quilt of suburbs and plains; around you the forest is cool and wet, stringybark and messmate with tree ferns in the gullies; but immediately behind you, just over the crest, the country is already drier and scrubbier again. What are you reading in this view, and why is the driest ground right behind the summit rather than out at the transect's far end?

Cues: A bright thread of gulf water beyond a quilt of suburbs and plains to the west · Cool, wet stringybark-and-messmate forest with tree ferns right around the lookout · Noticeably drier, scrubbier country already visible immediately over the crest behind you · No permanent stream or river valley in view — the vegetation change tracks height and aspect, not water flowing downhill

A cross-section of the whole gulf-to-range gradient stacked in order — gulf, plains, range front, wet summit forest — with a rain shadow tipping the country straight back to dry mallee just over the crest, so the wettest point is this high middle, not the far endA simple ramp of rainfall that keeps rising the further inland you travel, so the driest country lies far to the east beyond the rangesA river valley gradient, with the summit forest fed by a permanent stream that carries water down to the gulfJust scenery — the vegetation changes are scattered and don't follow any consistent pattern

The summit view stacks the whole gulf-to-range transect in the order it occurs on the ground, because everything below has organised itself around one changing ration of rain — from a few hundred millimetres at the dry coastal plain to roughly double that at the summit, one of the widest rainfall swings you can drive across in twenty minutes anywhere in the country (Gill 2014). But the ranges do not just catch that rain, they steal it: the country immediately behind the crest is thrown into a rain shadow and tips straight back down into mallee, so the wettest ground on the whole transect is this high middle, not its far end — a lopsided arch rather than a simple ramp.