When examined on a map, not even the dividing sea can conceal the evident geological continuities on either side of the Channel. A symmetry emerges in the physical landscapes, which succeed each other from west to east, culminating in the denuded anticlinal dome between the Weald and the Boulonnais.

Structurally and geologically, the cross-Channel region straddles the divide between two of Europe's constituent physical regions: to the west the older, much eroded, hercynian massifs – overlying a Pre-Cambrian basement - of the western peninsulas; to the east the newer sedimentary basins and the prolongation of the north European plain. To the southeast, both these regions were, in turn, affected to varying degrees by the most recent mountain-building phase during the mid-Tertiary period. These remoter earth movements – “the outer ripples of the Alpine storm” – produced a gentle flexing of the sedimentary strata which, through differential erosion, would translate into the classic “scarp and vale” topography. Much later, the Quaternary ice sheet to the north would barely touch the region, other than to leave an extensive ring of eolian-blown loess deposits along its southern rim.

The denuded ruins of an old mountain system found in the two western peninsulas include large masses of granite. In Cornwall and Devon they break the surface as exposed and rounded batholiths, in Brittany as long, straight ridges following the lines of the Armorican folding. But in both areas the greater part of the surface is made up of ancient sediments – slates, sandstones and grits of differing resistance. On the English side the gentle folding along a broad east-west synclinal axis recalls that of the Chateaulin Basin on the French side. Everywhere, prolonged and repeated cycles of erosion in the past have significantly reduced average altitude.

Higher rock cliff-faces on the north-facing flanks of both peninsulas result from tectonic tilting at the end of the Tertiary period, bringing about subsidence and a general rise in sea-level. Deeply indented coastlines are the result, drowned estuaries or “rias” (abers in Breton), nowhere better seen than on the western tip of Finistère. But for the protection offered by the cross-faulted, cross-shaped Crozon Peninsula, the great inlet between the Pointe St-Mathieu and Pointe du Raz would be even larger. Even so, its northern component, the Rade de Brest, forms the greatest natural harbour in western France. The largely granitic headlands, sometimes rising perpendicularly to a height of 100 m, provide a particular characteristic of the Breton coastline, with Lands End something of an exception on the English side. In the Lizard Peninsula, remnants of the oceanic crust forming the zone of contact preceding the building of the Hercynian mountain chain, come to the surface, the only such site in northwest Europe.

The eastern half of the cross-Channel region is best described as undulating lowland, where lines of low hills and open expanses of plateaux are separated by broad valleys. Structurally configured around a series of synclinal basins, each is given a degree of geographical unity by their drainage systems and connected across the Channel by geological history.

The rhythmic altercation of scarp and vale and the close traditionally-observed association of rock type and cultural landscape have identified regional entities as richly varied as the Weald of southeast England and the Paris Basin of northern France. In the latter region chalk forms the underlying rock of large areas in the north, where surface features and agricultural potential depend in large measure on the nature and extent of the superficial geology. Most important are the extensive tracts of loess deposits (limon) across northern France (with outliers as far as the north coast of Brittany, as well as along parts of the south coast of England). The "pays" most closely associated with these fertile surface loams, Beauce, Brie, Picardy and Artois, remain amongst the most renowned wheatlands in Europe.

The physical breach that brought about the Channel, and the permanent separation of the British archipelago from mainland Europe, is of a relatively recent date. Late on during the last Ice Age, within prehistory, there was already a long gulf reaching in from the Atlantic to form what is now the western part of the Channel. New interpretation of high resolution sonar images collected over the past 20 years has revealed the existence of a huge palaeovalley, over 10 km wide and up to 50 m deep along the whole length of the upper Channel sea-bed. The immediate cause was a megaflood unleashed by the sudden breach of the Weald-Artois Ridge ponding back the vast fresh-water, glacier-fed lake to the north. The devastating surge of water gouged the land beyond, creating a deep scar in the chalk bedrock of the Channel over 400 000 years ago. This was probably the first of two such megafloods in the aftermath of successive glaciations, a by-product of which would be the rerouting of the hitherto combined Rhine-Thames river system through the Channel.

Following the steady rise in sea-level since the last Ice Age, the English Channel has witnessed significant changes in its coastline, an evolution recorded in countless marine charts over the last 500 years. These movements of the coast represent variations on the theme of submergence, largely resulting from two processes – erosion and deposition. Clearly, marine erosion down Channel has been slowed by the greater resistance of granitic headlands and other harder outcrops of palaeozoic date. By contrast, up Channel, where coastlines are sculpted out of less resistant chalk and other sedimentary strata, the “hanging valleys” or “valleuses” of East Sussex and the Pays de Caux testify to the power of wave action and rapid rate of coastal retreat.

The example of the Cotentin Peninsula reveals a different combination of processes at work. With its rectangular form partly due to the faulting that produced the Channel Islands, also bringing to the surface the oldest rocks in western Europe, it was once itself an island, later linked to the mainland by uplift. Subsequent submergence common to the whole Channel area has resulted in low-lying marsh across the neck of the peninsula. More generally, the filling in of embayments and estuaries has led to a significant redrawing of the coastline in historical times, as exemplified on both sides of the Channel around the Dover Strait, where the former shoreline is marked in part by lines of abandoned cliffs. Today Dungeness is the greatest shingle foreland on the English coast, constantly shifting under the remorseless influence of the tides, part of a longshore, generally eastward drift of material.

Throughout the medieval period, this changing coastal geography would influence the evolution of Channel ports. With the progressive silting up of the harbours of the Cinque Ports (Kent-Sussex), for example, the balance of advantage would move down Channel given the greater stability of its coastline and the safety of its protected, deep-water anchorages, closer to the open sea.