DALEHOLES NORTH OF TWISK

Daliegaten are more worth knowing than worth seeing, they are there but you (often) don't see them. It is plausible that dalie holes were dug on a large scale north of Twisk, as elsewhere in West Friesland.
In (former) peat bogs, calcareous sand and clay were extracted from deep holes, which were dug through the peat layer into the underlying clay. The calcareous sand and clay were mainly used to improve the peaty soils to make grain and hemp cultivation possible.
Digging a dale hole must have been quite an undertaking at the time. Above the clay was a layer of peat of at least 3 to 4 meters thick. The pits were dug to about 2 meters into the clay soil. The dale holes originally had a depth of about 5 to 6 meters. This means that in the case of dale holes with a diameter of 2.5 metres, approximately 10 m³ of clay has been extracted per hole.
After the calcareous material had been extracted, the resulting holes were filled with excess peat. In areas where the peat layer has disappeared (due to excavation, oxidation or erosion), the backfills of the dalie holes are the only remnants of the former peat package. The muddy filling of the holes sinks faster than the surrounding area, so that after some time the dale holes become visible as small round 'depressions' in the landscape.
The holes have a diameter of 2 to 5 meters, but some of them are as long as 5 to 7 meters. If the surface is not levelled, they are generally 20 to 50 cm deep. In wet periods, precipitation often collects in the dalie holes.

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