Simplify the following text for a Grade 4 native English-speaking person.
Original: Most tangible evidence of this are the dolmens ( hunebedden ) built around 3500 BC , 53 of the 54 dolmens in the Netherlands can be found in Drenthe , concentrated in the northeast of the province .
Simplified: Around 3500 BC people made dolmens ( hunebedden ) , piles of big stones . 53 of the 54 dolmens in the Netherlands are in Drenthe .