File:The Quarterly journal of the Geological Society of London (14072583161).jpg

IN THE CHALK WOLDS OF LINCOLNSHIRE.
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the north. This little stream, instead of continuing its southerly
course, turns abruptly to the east and enters a narrow ravine exca-
vated out of the Chalk, which extends for a distance of about half a
mile and then opens into the main valley opposite the village of
South Thoresby (see fig. 1).
Fig. 1 . — Plan of the country near Swahy. (Scale 1 inch to a mile.
The ground occupied by Drift is indicated by diagonal shading.)
The sudden change in the form of the valley below Swaby is
very striking. Above the village, the valley-bottom is some 150
yards wide, and the valley-sides have the usual gently sloping
outline of chalk hills. Below the village the valley is contracted to
a trench-like ravine, the bottom of which is perhaps 40 yards wide,
and its sides rise in steep slopes, the angle being in many places that
of chalk debris (viz. 35°-38").
The wider and older valley is actually continued southward
through the village for some distance, the barrier of drift being about
400 yards beyond the point where the stream turns aside to enter
the ravine. This modern continuation of the watercourse is clearly
therefore a new cut made through the solid mass of chalk, which
originaUy formed one flank of the ancient valley.
It would seem that on the cessation of glacial conditions the
stream was able to occupy its former valley as far as Swaby, but
being there ponded back it made its way over a col in the chalk hiUs
which was lower than the surface of the drift barrier, and having
once taken this course it would naturally maintain and deepen the
new channel, forming the ravine above described.
The glacial clays and gravels also occupy for some distance the
valley of another tributary which joins the Calceby Beck south of
Swaby. This stream also has cut into the chalk and runs in a small
ravine for a distance of about three furlongs ; but it is only just out-
side the limits of the drift area and is not so striking as the valley
first described.
There can be little doubt that the ancient courses of the two.
brooks are concealed beneath, the glacial deposits ; they probably
united somewhere near Swaby church, and thence the combined