forest rings
... that forest rings had gone unnoticed by geologists until aerial photography became a common surveying tool in the 1950s?
... that <b id="mwB0A"><a href="./Forest_ring" id="mwB0E" rel="mw:WikiLink" title="Forest ring">forest rings</a></b> had gone unnoticed by geologists until <a href="./Aerial_photography" id="mwB0I" rel="mw:WikiLink" title="Aerial photography">aerial photography</a> became a common surveying tool in the 1950s?