Upper and Lower Resevoirs of the Dodder are situated in a glaciated valley.
This valley was created by a glacier. Before the glacier moved into position
it had collected boulders, rocks, stones, pebbles, gravel, sand and clay.
These were embedded in the ice of the glacier and as it moved eroded the
underlying rock. The glacier was in the valley for thouands of years and
while it was there it slowly moved downwards and sideways. This caused
the sides of the valley to become steeper
and the bottom to become wider.When the ice melted there was a steep -
sided u-shaped valley with a lot of glacial till left behind. The Dodder
began to cut its way down through the glacial till.The engineers in the
19th century thought this was an ideal place for a resevoir.