Megalithic builders almost always settled in hilly country, and in the Netherlands they settled on the higher and naturally drained land in the east. In the province of Drenthe big boulders are plentiful because it is a glacial moraine, and it is here that the hunebedden are found. These collective burial chambers are partly sunk into the soil, with a row of trilithons: two uprights roofed by a capstone. Their length varies from 4-24 meters, and the entrance is usually on the southeast. It is assumed that the earliest hunebedden go back to about 3400 BC. In all 54 have been discovered so far.