The Lake is also divided into two unequal portions by the straits of
White Horse, where vessels are often weather-bound, and cannot make way
against the wind, which sets a current through the narrow channel. There
is no tide; the sweet waters do not ebb and flow; but while I thus
discourse, I have forgotten to state how they came to fill the middle of
the country. Now, the philosopher Silvester, and those who seek after
marvels, say that the passage of the dark body through space caused an
immense volume of fresh water to fall in the shape of rain, and also
that the growth of the forests distilled rain from the clouds. Let us
leave these speculations to dreamers, and recount what is known to be.

For there is no tradition among the common people, who are extremely
tenacious of such things, of any great rainfall, nor is there any
mention of floods in the ancient manuscripts, nor is there any larger
fall of rain now than was formerly the case. But the Lake itself tells
us how it was formed, or as nearly as we shall ever know, and these
facts were established by the expeditions lately sent out.

At the eastern extremity the Lake narrows, and finally is lost in the
vast marshes which cover the site of the ancient London. Through these,
no doubt, in the days of the old world there flowed the river Thames. By
changes of the sea level and the sand that was brought up there must
have grown great banks, which obstructed the stream. I have formerly
mentioned the vast quantities of timber, the wreckage of towns and
bridges which was carried down by the various rivers, and by none more
so than by the Thames. These added to the accumulation, which increased
the faster because the foundations of the ancient bridges held it like
piles driven in for the purpose. And before this the river had become
partially choked from the cloacæ of the ancient city which poured into
it through enormous subterranean aqueducts and drains.