The Humber Gap is a term for the geographic gap between the roughly north-south running line of hills formed by the Yorkshire Wolds and the Lincolnshire Wolds, formed by the west-east running Humber estuary.
In the geological past the gap has formed part of an ice barrier due to glaciers during the ice age resulting in damming and formation of a 'Humber Lake', and also forms a geological division.
In modern times the gap has formed a natural choice for transport routes, such as the railways. The Humber Bridge also crosses the Humber close to the gap.
References
Humber Gap Wikipedia(Text) CC BY-SA