Hi Petra. That field of rocks is interesting. Those rocks are known as volcanic bombs. Volcanic bombs are lava fragments that were ejected while viscous (partially molten) and larger than 64 mm in diameter. Many acquire rounded aerodynamic shapes during their travel through the air. Volcanic bombs include breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs, and "cow-dung" bombs.
You’re lucky to have found the blackberries. I have to fight a Blue Jay for the ones we have in our backyard. Take Care…Don in creepy town.