Table 1 Summary of geomorphological features associated with ice shelf presence and absence^{30,83,84,91,93,112,149,150,151}.

Grounding zone wedges (GZW) [22]

Sedimentary wedges formed of soft diamicton (till), with an asymmetric cross-sectional geometry; internal ‘prograding’ reflectors and truncated topsets

Up to 15 km long, several metres to 100 m thick

Quasi-stable period when the ice margin remains near-stationary allowing the accumulation and forward progradation of a sedimentary wedge. Vertical restriction and elongated ‘wedge’ cross-profile indicate limited accommodation space due to the presence of a seaward floating ice tongue

Pine Island Trough^93,149

Melange scours [23]

Straight and sub-parallel, down-wardly incised linear landforms sometimes with dog-legged kinks along their tracks (Fig. 6a)

1–10 km long, ~100 m wide, and 2–10 m deep

Keel-mark incisions from a free-floating mass of icebergs held within an ice melange. The forward motion of the melange is aided by a floating ice shelf forming the seaward extension of an ice stream upstream

Filchner Ice Shelf, Weddell Sea⁹¹

Sub-ice shelf keel scours [24]

Arrays of splayed and cross-cutting lineations with high linearity but low parallel conformity, typically on the surface of a GZW. In contemporary settings, scour terminates abruptly at a positive asymmetric mound

~100–400 m spacing/width, 1–10 km long, 1–10 m high

Scours formed as a marine ice stream comes afloat in the grounding zone. Terminal mounds created by sediment piling ahead of the forward-moving ice- shelf keel

Outer Pine Island Trough, West Antarctica¹⁴⁹; Modern Pine Island Glacier ice shelf cavity⁴⁰

Corrugation ridges [25]

Small-scale transverse ridges, within or overprinting linear scours

0.5–2 m high, 70–150 m crest spacing, possible cyclicity in amplitudes

Expression of tidal lifting and settling of the ice shelf.

At Pine Island Glacier, corrugations are interpreted as formed under the intact ice shelf based on: (1) observations that the sub-ice shelf ridge is a regular site of modern unpinning and regrounding by deep-drafted ice keels

(2) Convincing sediment core evidence for full ice sheet grounding on the top of Jenkins ridge as recently as 1940s, and subsequent cavity opening, implying the surface morphology of the Jenkins Ridge is a fresh imprint of ungrounding. (3) A lack of direct geological evidence that the Pine Island ice shelf was absent during previous warm times of the Holocene, required to explain the corrugations as iceberg-formed features (although see ref. ¹⁴³)

Modern Pine Island Glacier ice shelf cavity³⁰

Ice-shelf moraines

Gently inclined shore or ice-shelf edge parallel ice- cored linear debris accumulations formed around ice shelf edges

—

Originate from thrusted slabs of glacimarine sediment, folded debris- rich basal ice, and/or the accretion of sea-water and basal marine sediments. Sometimes contain marine organisms that can be dated

George VI Ice Shelf⁸³; McMurdo (Minna Bluff), Sorsdal Ice Shelf⁸⁴

Iceberg ploughmarks [26]

Cross-cutting, curvilinear to sinuous scour marks. V- shaped in profile often with flanking berms. Sometimes terminate in rimmed pits. Single or multi-keeled expressions

10–>200 m wide, 100 s of m to >30 km in length, 1–>30 m deep

Ploughed grooves formed by scour from iceberg keels. Occur in a number of settings but expected to be abundant on seafloor landscapes shaped by ice shelf retreat. Cut-off in population depths potentially indicative of MICI processes

Pine Island Trough¹¹². Norwegian margin¹⁵⁰

Ice-plough ridges [27]

Crescentic mounds at the termini of linear ice-keel furrows (Fig. 6b, inset)

<1 km long, 50–~200 m wide, up to ~20 m high

Formed by sediment pushing due to the multiple grounding of mega- icebergs during an ice-shelf breakup. Ridges or pits form prior to rotation and ungrounding of the berg keel

Mid-Pine Island Trough⁹³; Northern Barents Sea¹⁵¹

Mega-berg furrows [28]

Down-wardly incised deep, linear to slightly curved scours, sub-parallel

2–10 km long, 5–>20 m deep, 150–> 500 m spacing

Produced by deep keels of large and thick icebergs entrained in ice melange formed by rapid ice-shelf breakup

Mid-Pine Island Trough⁹³; Northern Barents Sea¹⁵¹

Corrugation ridges [25]

Small-scale, transverse ridges, forming tracks within or which overprint linear scours

0.2–2 m high, 35–200 m crest spacing, reduce in spacing down-flow.

Amplitudes vary systematically along flow

Formed at the trailing edge of mega-bergs entrained in a coherent proglacial ice melange, retaining an expression of tidal lifting and settling of berg keels.

Cyclicity in amplitude consistent with modulation by tides in open water, with one corrugation ridge forming per day by the gradual rising and settling of the iceberg keel on seabed sediments.

The spacing of the ridges which reduces down-flow suggests the icebergs slowed as they drifted from the retreating ice face and grounded on the seaward-shallowing seabed

Mid-Pine Island Trough⁹³; Northern Barents Sea¹⁵¹