_North of the Somme._--The battle zone, bounded by the rivers Ancre,
Somme and Tortille--the latter doubled by the Northern Canal--forms a
strongly undulating plateau (altitude 400-520 feet), which descends in
a series of hillocks, separated by deep depressions, to the valleys of
the rivers (altitude 160 feet). The Albert--Combles-Péronne railway
runs along the bottom of one of these depressions.
The higher parts of the plateau form a ridge, one of whose tapering
extremities rests on the Thiepval Heights, on the bank of the Ancre.
Running west to east, the ridge crosses the Albert-Bapaume road at
Pozières, passes Foureaux Wood, then north of Ginchy. It is the
watershed which divides the rivers flowing northwards to the Escaut and
southwards to the Somme.
The second line of German positions was established on this ridge,
while the first line extended along the undulating slopes which
descended towards the Allies' positions. There were other enemy
positions on the counter-slopes behind the ridge.
These positions took in the villages and small woods of the region,
all of which, fortified during the previous two years, bristled with
defence-works and machine-guns.
Some of these villages (Courcelette, Martinpuich, Longueval, Guillemont
and Combles), hidden away in hollows, were particularly deadly for
the Allies; the defenders, unseen, were able to snipe the assailants
as they appeared on the hill tops. The Allies had to encircle these
centres of resistance before they were able to enter them.
_South of the Somme._--The battle zone, bounded by the large circular
bend of the Somme at Péronne, formed a kind of arena. The vast, flat
table-lands of the Santerre district, separated by small valleys,
descend gently towards the large marshy valley of the Somme, in which
the canal runs parallel with the river.
Owing to the narrowness of this zone, the Germans were forced to
establish their positions close behind one another, and the latter
were therefore in danger of being carried in a single rush. On the
other hand, the assailants' rapid advance was first hampered, then held
by the marshy valley, which prevented them from following up their
brilliant initial success.
During the battle, the Germans, driven from their first positions,
hastily prepared new ones, and clung desperately to the counter-slopes
of the hills which descend to the valleys.