Only few dolmen are excavated or otherwise preserved. The pottery sherds are lying on the surface because agricultural and probably other vehicles are milling their way through these ancient grave yards. Portugal's archaeology must be in desparate need of money; it's a shame because there are probably enough young but jobless archaeologists like elsewhere.
Looking at the bright side: Nature was flowering everywhere. Maybe I missed the O-Hanami this year but we certainly saw other trees in flower like this rosaceae species.
I think it is a Cydonia oblonga (quince or Quitte in German). Although the leaves seamed softer than the one we have in Germany. Therefore I don't bet my life on it. If anyone can specify it with certainty - let me know.