The 1:1,000,000-scale Quaternary surficial deposits map of the Arabian Shield is a synthesis of the 56 1:250,000-scale geological sheets covering the shield. More than 50 different types of surficial deposits or groups of deposits ranging in age from Pleistocene to Holocene were taken into account and divided into the following six units: alluvium, depression deposits, sand, gravel, undifferentiated sand and gravel and undifferentiated deposits.

These surficial deposits are ubiquitous throughout the Arabian Shield but are particularly abundant along the Red Sea coast and to the east and north, in the depression between the Proterozoic basement and the Phanerozoic cover. Where the deposits significantly mask the underlying bedrock, they are mapped as total cover (90-100% surficial deposits). Where they are either lacking or represented in insufficient detail on the source maps to be reported at the synthesis scale, they are mapped as partial cover (0-10%, 10-50% or 50-90% surficial deposits).

The description of these six units is summarized from the explanatory notes of the 56 maps that were used.

Alluvium (Qa) comprises wadi bed deposits filling the main channels of the active drainage system, present and ancient flood-plain and basin deposits, terrace deposits with or without pediment, and alluvial-fan deposits.

The channels of the active drainage system are filled with unconsolidated, crudely bedded to well-stratified gravel, sand, silt and scattered boulders, subangular to well rounded, and locally including reworked loess or scoria derived from volcanic extrusions. The deposits are generally well defined, with crossbedding and channel structures, but certain are extremely diffuse and overlapped by eolian sand. Towards the coast, the sedimentary load of most wadis becomes distributed as deltas over sabkhahs and tidal mudflats, with only a few maintaining a channel as far as the open sea. Most active wadis of the Proterozoic domain, and those of the fossil system in the eastern depression, are too narrow to be represented at the synthesis scale where they are not associated with extensive flood-plain or terrace deposits; they are thus assigned to a different unit.

The flood-plain and basin deposits of the main present channels and certain fossils channels are covered by silt, clay, sand and subordinate gravel, locally interlayered with boulder beds, sand bars, overbank deposits or reworked loess. In the Jizan quadrangle, they are correlated to the north with the alluvial terraces of the Sabya quadrangle, thought to be Pleistocene to Holocene in age.

The terrace deposits consist of silt, sand, cobbles and gravel and may or may not show a desert pavement and/or a desert varnish, and locally a well-defined red soil horizon. In some places, several levels of terraces are present above the present-day channels. In the Wadi Baysh quadrangle, they are thought to be Pleistocene in age.

Alluvial fans occur at the foot of the Red Sea escarpment and along certain larger wadis. The deposits consist of an unconsolidated, poorly sorted talus of pebbles and boulders, with intercalated lenses of pebbles and sand, formed by recent active erosion of the escarpments.

Depression deposits and duricrusts (Qb) include sabkhah, khabra and lacustrine deposits, duricrusts and reef complexes.

This unit is present along most of the Red Sea coast where it predominantly comprises sabkhahs and tidal mudflats. These deposits consist of silt, mud and clay interstratified and encrusted in the sabkhahs by halite and gypsum. On their landward side, the deposits are locally covered by alluvium or eolian sand, whereas on their seaward side, they are protected from coastal erosion by an outer line of beach dunes or reef complexes. The latter consist of submarine, actively growing reef limestone and uplifted, inactive reef-limestone terraces, rising a few metres above sea level, of calcareous beach sand deposits, arenaceous limestone and calcareous breccia ("beach rock"). In the Jiddah quadrangle, a reef limestone gave a 14C isotopic age of 40,000 years BP.

Many small patches of sabkhah, khabra and lacustrine deposits occur in the Proterozoic domain, but few are mapped at the present scale. These deposits are predominantly developed in the eastern depression where they are generally associated with a network of fossil alluvial channels that drained the basement and the Paleozoic escarpment. Sabkhah and lacustrine deposits also commonly occur in interdunal depressions. Most of these deposits are thus inactive. The khabra deposits consist of clayey-silty playas or sandy material, the lacustrine deposits of clayey material, carbonates and sandy silt locally containing abundant gastropod or bivalve shells, and the sabkhah deposits of silt and clay with interbedded evaporitic deposits.

Duricrusts or thin hardened layers resembling duricrust are commonly developed over several rock types of the eastern Phanerozoic cover. Two episodes of encrustation are recognized and are the result of a long and complex evolution during the Early Quaternary and possibly the Late Pliocene.

Sand deposits (Qs) predominantly include eolian sand fields, and locally, beach sand or extensive loess silt, as along Wadi Bishah (Wadi Bishah quadrangle).

Although some extensive sand sheets occur in the Proterozoic domain with well-developed dune fields, the sand deposits are essentially represented in the eastern and northern parts of the shield where they form large sand sheets and dune systems. The sand sheets vary in thickness ranging from a thin film overlying gravel-plain deposits or older layers, to deposits several metres thick. The dune systems comprise extensive mobile or fixed dune fields. Longitudinal dune chains may be more than 1 km wide and as much as 35 km long, separated by interdunal belts. Most are oriented northwest, others are northeast- or east-trending, with their direction not always corresponding to the present prevailing winds, and possibly reflecting at least two periods of aridity.

This unit also covers most of the coastal plain where it comprises sand sheets and tracts of dunes, and extensive sheets of wind-blown material deposited on loess, old flood-plain deposits and pediment deposits. The unit can form east-trending linear dune ridges of 1 to 5 m high, 15 to 100 m wide and as much as 1 to 5 km long, separated by inter-dune areas. These locally grade into tracts of barchan dunes, not exceeding 10 m wide and 2-3 m high, which are generally open to the east or southeast. In some places, dunes encroach upon wadi channels or sabkhahs where sand is reworked.

All these sand deposits consist of fine- to medium-grained quartz locally mixed with abundant feldspar, heavy minerals or other material; most are Holocene in age, but some may be Pleistocene.

The wind-blown material consists of silt to medium-grained subangular to subrounded sand, and is generally thin (less than 1 m) with surfaces marked by ripples or small dunes.

Gravel sheets (Qg) include vast expanses of sand and gravel, colluvium, and pediment material derived from Proterozoic outcrops or developed on an unidentified unit.

The gravel sheets are generally vast, containing blocks all of which are derived from older deposits, and form flat glacis dipping gently towards the depression. To the north and east, where they are well-developed, the gravel sheets can be divided into three episodes: 1) an old sheet, without recent volcanic rocks, thought to be Early Pleistocene, 2) an inactive sheet, containing recent volcanic rocks, ranging in age from early Late Pleistocene to Holocene, and 3) an active sheet mainly corresponding to present piedmont deposits.

Everywhere, the colluvium comprises detritus formed by weathering of the underlying bedrock and transport by gravity and runoff over short distances. The deposits consist of unsorted angular rock fragments of all sizes forming fan-shape accumulations. Abundant colluvial deposits occur at the foot of and on the dip slopes of the Khuff Formation cuestas in the eastern part of the shield.

The pediment material consists of a thin veneer of poorly sorted, fine- to coarse-grained gravel, sand and silt, partially covered by or merging with wadi sediments and eolian sand.

Most of the surficial deposits of the shield belong to this unit, being predominantly composed of piedmont and pediment deposits, but also including alluvium at the bottom of the thalwegs and old alluvial terraces entirely eroded. The latter case is well illustrated by the extensive sandy plain of Sahl Rukbah (Turabah quadrangle).

Undifferentiated sand and gravel deposits (Qsg) comprise eolian sand fields, with or without dunes, gravel sheets, and pediment material. In the Al Qunfudhah quadrangle, this unit also includes sabkhahs along the coast.

Undifferentiated deposits (Q) comprise unidentified Quaternary surficial deposits and a mixed unit formed by deposits of various origin that cannot be differentiated at the present scale of work.