Summary of site:
The Braad Dry Valley (a valley without a stream) is one of several in the area. They were cut either at the end of the last major glaciation, when the ground was completely frozen and only surface streams could flow, or when a much higher water table prevented any substantial underground flow. In areas of pure limestone (such as the Dartry Limestone here), there is always a tendency for water to seep into fissures widened into substantial passages and caverns by chemical weathering. This means that a valley cut by normal surface flow can be left high and dry as the water table falls.