Hunebedden building-instructions

How did the simple farmers that inhabited Drenthe 5500 years ago manage to transport 20 ton boulders to a construction site and pile them up to form burial chambers..? This drawing illustrates a possible way. (thought of in 1857 by King Frederic VII of Denmark) 

First of all they selected boulders found in the neighborhood with a flat side. Some of them had to be dug out from the sand first. The stones were placed on wooden rollers or sleds by means of levers. Manpower and oxen did the rest. The possibility has been proven by tests: 30 strong men were able to move these heavy boulders in this way. 

The construction began by building a earthen dam. Two rows of standing sidestones (uprights) were placed in pits against the dam, he flat side inwards and secured by cobbles and sand. Both ends were closed by two endstones. Next was the building of an inclining hill of sand. The heavy capstones, the flat side downwards, were dragged to the top of the hill and placed on top of the sidestones. After completion the skeleton of the burial chamber was ready and the sand within could be removed.

The openings between the stones were filled up by smaller boulders and dry masonry of flattened stones. On the south side an entrance was constructed by 2 or 4 passage sidestones and a passage-capstone. The floor of the chamber was paved by small pebbles and grit. Finally the completed chamber was covered by a barrow consisting of sods, sand and cobbles, leaving only the top of the capstones visible. Sometimes the foot of the barrow was strengthened by a oval wreath of smaller boulders (kerbstones).