Megalith

A megalith is a large stone used to build a structure or monument, either alone or together with other stones. Megalithic means structures made of such large stones, put together without the use of mortar or cement. 
The word "megalith" comes from the Ancient Greek ' "megas" meaning "great", and ' "lithos" meaning "stone". It has been used to describe stone structures built by people from many parts of the world living in many different times. Many megaliths were used to figure out the dates of the solstices and equinoxes.
These structures were built mainly in the Neolithic period. They continued to be built into the Bronze Age. Some were built even earlier in the Mesolithic period.
A famous example of a megalith is Stonehenge.