The principle tribes of the English were Angles, Lower Saxons and easy to forget the most northerly of the English tribes, the Jutes. All of them had settled to agricultural along the North Western shores of Europe. There are some who will tell you with vehemence that there is actually no difference between the Jutes from Jutland and the Geats from the rocky and often cold shores of southern Sweden. Perhaps more interesting is why did the Geats hang on in southern Sweden despite incursion by the Goths, and why did the Jutes from the flatness of Jutland cross the North sea to take land from Romanized Island Celts, or The Britons. The answer can be found in what the professionals call "A Marine Transgression," and what I prefer to think of as "Sea swallowing up land so that people can't live there anymore." Worth remembering there are others who will tell you that seven or nine thousand years ago, about when Jericho had it's first city wall, if you lived in Jutland you could probably have walked across the North Sea to Hull.
But, more specifically for the principal tribes of the
English, it was the "Dunkirkian Marine Transgression," that mattered. This
transgression had a number of phases, one of which was called "Dunkirkian II."
It lasted from around 300AD until around 600AD. It eroded away or swamped good
portions of the lowland in North Western Europe. Other "Dunkirkian
Transgressions," had no effect upon the Jutes, the Angles and the Lower Saxons,
because after about 500AD they had become Anglo Saxons, who all lived and had
their being east of a line from about the River Tweed to the mouth of the River
Severn, then along the coast a bit to around Barnstable, then south across land
to Exeter. The foot of Cornwall was where the Dumnonii Celts remained stubborn.
The Jutes themselves moved as southward as possible, they dominated Kent and
Hampshire. But in the various Celtic dialects all these European invaders were
called "Saeson" or "Sasannach." And the thing the Sasannach had in common
was a developing language that came to be called English, which is these days
first spoken by almost as many people as first speak Spanish. Spanish, around
the time of the Sasannach incursion upon the Celts of Britain, was a dialect of
Latin spoken in the Iberian Peninsular, or Spain.