The formation of the gorge section of the
Vistula valley below
Fordon is linked to the development of the drainage system around
Bydgoszcz at the transition from the Upper Plenivistulian to the Late
Glacial period. At the end of the last glaciation, as the ice front retreated across northern Poland, meltwaters from northern
outwash plains of rivers such as the
Skrwa,
Drwęca,
Brda,
Wda, and
Gwda merged with waters flowing from the south, forming a massive proto-Vistula current. This current carved an
urstromtal several kilometers wide, flowing westward toward the
North Sea. The lower Vistula gorge occurred when these waters found a shorter outlet, redirecting flow through a
tunnel valley in the lower
Powiśle region, which concealed a buried valley from earlier geological epochs. The formation of the
Lower Vistula Valley and the redirection of glacial waters occurred in a three-stage process. The classical explanation is based on analyzing
fluvial terrace systems and their ages in the southern part of the valley ( and
Fordon Valley), where a complete nine-level terrace system is preserved. However, newer studies suggest that the terrace model does not fully explain the valley's formation. Evidence indicates that
dead ice also played a role, with some presumed terrace levels having a
kame origin.
Proglacial phase During the proglacial phase, primarily associated with the
Pomeranian phase, meltwaters, after depositing
outwash plain fields, formed valleys within them, flowing south toward the developing . In the
braided flow of the urstromtal, meltwaters mixed with southern Polish river waters, heading westward. From
Chełmno to
Fordon, an outwash plain valley of the
Wda and
Mątawa rivers formed, channeling meltwaters from the Tuchola outwash plain. This valley evolved into a proglacial valley, serving as the initial form for the Fordon Vistula gorge. According to R. Galon, meltwater drainage southward to the occurred at the level of terraces IX–XI (83–86 m above sea level). This southern drainage likely persisted during the Pomeranian phase, as indicated by the development of the Wda and Mątawa outwash plains.
Transitional phase Following the retreat of the
ice sheet from the Pomeranian phase to northern Pomerania and the southern Baltic Sea, the second developmental phase, known as the
bifurcation phase, began. During this period, some glacial waters near Fordon, at the level of the high terrace IX (60–61 m above sea level), likely during a flood surge, redirected into the proglacial valley near Chełmno and began flowing north toward the
Gdańsk Bay. The trigger for this redirection was the melting of
dead ice buried in the glacial deposits of this
tunnel valley. Although the Wda outwash plain valley was utilized by some Vistula waters, most continued flowing westward through the . Researchers date the start of the bifurcation phase to 14,000–12,000 years ago. According to R. Galon, the bifurcating waters flowed at the levels of terraces IX, VIII, and VII (50–61 m above sea level), lasting about 1,000 years. Newer studies indicate that bifurcation from the occurred only at terraces VIII and VII. The initial northward river flow from the Toruń Basin occurred at a terrace level 30–32 m above the current Vistula level (58–60 m above sea level).
Baltic phase The third phase of the valley's development occurred when the ice sheet was confined to
Scandinavia. The valley's formation was driven by a low erosion base (the level of the emerging Baltic Sea), leading to rapid vertical erosion of the valley and the cessation of westward flow in favor of a shorter northeastern route to the
Baltic Ice Lake. As a result, the Brda waters entered the abandoned
urstromtal between and , carving new
fluvial terrace edges. Intense vertical erosion of the urstromtal floor in Bydgoszcz correlated with the lowering Vistula level. A
drainage divide formed between Bydgoszcz and
Nakło nad Notecią, separating the
Oder and Vistula river systems, while the urstromtal west of Bydgoszcz became swampy and peat-filled. The exclusive northward flow of the Vistula occurred 12,000–11,000 years ago, during a warmer
interstadial period, at a terrace level 16–17 m above the current Vistula level. This is evidenced by the presence of this terrace along the lower Vistula, absent in the urstromtal west of the Brda. The further development of the Lower Vistula Valley is reflected in the formation of the terrace system from VI to I. Initially, rapid valley deepening occurred due to the low erosion base of the Baltic (
Yoldia Sea,
Ancylus Lake, 10,000–8,000 years ago), followed by predominant river accumulation during the
Littorina Sea period (8,000–4,000 years ago), which deposited an
alluvial cover up to 20 m thick on the valley floor. == Characteristics ==