MAPS AS GEOGRAPHICAL ILLUSTRATIONS 507 
from a side valley on the south about ten miles west of the 
elbow at Bromberg. This district has been cited by various 
authors as offering a good example of the diversion of a river 
from an old toa new course, leaving the old course to be occu- 
pied by a stream that is too small for it; but the cause of the 
diversion is not clearly stated. It is probably connected in 
some way with the changes determined by the later phases of 
glacial action. 
On going up the flat valley- of the Netze, the plane of its 
gently ascending floor may be prolonged beyond Bromberg, and 
discovered again in terraces that now stand somewhat above the 
present channel of the Vistula. These terraces, like the meadow 
floor of the Netze, are enclosed by rather well defined slopes 
that descend from the rolling drift country of the upland. 
About four miles west of the elbow and close to the town of 
Bromberg, a small stream, the Brahe, enters the broad-floored 
valley from a narrow valley on the north. Instead of turning 
west and joining the Netze, it turns east and, as it were, flows 
backward to the Vistula, trenching the valley floor slightly on 
itsway. This behavior of the Brahe has very likely resulted from 
the formation by it of a flat alluvial fan on the broad floor of 
the Netze trough; such a fan being the characteristic product 
of a side stream that enters a broad valley deserted by the 
master stream. With the growth of the fan across the Netze 
trough, the Brahe has there formed a flat divide, and, happen- 
ing to run down the eastern slope of the fan, it found its way to 
the Vistula instead of maintaining a connection with the Netze, 
as it must have done for a time after the diversion of the greater 
river to its new course. 
It is important to note that the breadth of the Netze trough 
is not alone a sufficient reason for arguing that it cannot have 
been formed by a small stream ; for small streams may, in time, 
form broad valleys. But if the Netze had formed the broad 
valley, all the other small streams of the region should also flow 
in broad valleys, and this is by no means the case. Not only 
the Brahe from the north and the Netze from the south, but 
