Here is a definition and brief background on dolmens from wikipedia:

A dolmen, also known as a cromlech, portal tomb, portal grave or quoit, is a type of single-chamber megalithic tomb, usually consisting of two or more megaliths supporting a large flat horizontal capstone (“table”), although there are also more complex variants. Most date from the early Neolithic (4000-3000 BCE). Dolmens were typically covered with earth or smaller stones to form a tumulus. In many instances, that covering has weathered away, leaving only the stone “skeleton” of the burial mound intact.

It remains unclear when, why, and by whom the earliest dolmens were made. The oldest known dolmens are in Western Europe, where they were set in place around 7000 years ago. Archaeologists still do not know who erected these dolmens, which makes it difficult to know why they did it. They are generally all regarded as tombs or burial chambers, despite the absence of clear evidence for this. Human remains, sometimes accompanied by artifacts, have been found in or close to the dolmens which could be scientifically dated using Radiocarbon dating. However, it has been impossible to prove that these remains date from the time when the stones were originally set in place.^[1]

What do you think?  Were some of these made by softening of stone or is there another technique?  Feel free to comment.