▁century ▁or ▁earlier ▁when ▁they ▁were ▁built ▁to ▁divide ▁fields ▁and ▁retain ▁liv est ock . ▁Some ▁extremely ▁well ▁built ▁examples ▁are ▁found ▁on ▁the ▁lands ▁of ▁Much alls ▁Castle . ▁ ▁D ry ▁stone ▁walls ▁can ▁be ▁built ▁against ▁emb ank ments ▁or ▁even ▁vertical ▁terra ces . ▁If ▁they ▁are ▁subject ed ▁to ▁later al ▁earth ▁pressure , ▁they ▁are ▁retain ing ▁walls ▁of ▁the ▁type ▁gravity ▁wall . ▁The ▁weight ▁of ▁the ▁stones ▁res ists ▁the ▁pressure ▁from ▁the ▁retained ▁soil , ▁including ▁any ▁sur charg es , ▁and ▁the ▁fr iction ▁between ▁the ▁stones ▁causes ▁most ▁of ▁them ▁to ▁act ▁as ▁if ▁being ▁a ▁mon ol ith ic ▁gravity ▁wall ▁of ▁the ▁same ▁weight . ▁D ry ▁stone ▁retain ing ▁walls ▁were ▁once ▁built ▁in ▁great ▁numbers ▁for ▁agricult ural ▁ter rac ing ▁and ▁also ▁to ▁carry ▁paths , ▁roads ▁and ▁rail ways . ▁Although ▁dry ▁stone ▁is ▁s eld om ▁used ▁for ▁these ▁purposes ▁today , ▁a ▁great ▁many ▁are ▁still ▁in ▁use ▁and ▁maintained . ▁New ▁ones ▁are ▁often ▁built ▁in ▁gard ens ▁and ▁nature ▁conservation ▁areas . ▁D ry ▁stone ▁retain ing ▁structures ▁continue ▁to ▁be ▁a ▁subject ▁of ▁research . ▁ ▁Since ▁at ▁least ▁the ▁Middle ▁A ges ▁some ▁brid ges ▁capable ▁of ▁carrying ▁horse ▁or ▁carriage ▁traffic ▁have ▁been ▁constructed ▁using ▁dry stone ▁techniques . ▁An ▁example ▁of ▁a ▁well - pres erved ▁bridge ▁of ▁this ▁type ▁is ▁a ▁double ▁arch ed ▁lim estone ▁bridge ▁in ▁Al by , ▁Sweden , ▁on ▁the ▁island