Hot weather across Britain, it turns out, is a great time for archeologists to take to the sky.

Through aerial archeological surveys, hot and dry conditions have made it possible to see “cropmarks,” or shapes denoting ancient structures, from above. Cropmarks are darker areas of vegetation where fortification ditches from past settlements once were. Even though they were filled in many years ago, the topsoil reaches down deeper than the rest of the surrounding farmland. When hot and dry weather strikes, the vegetation above the filled in ditches has more nutrients and moisture, leaving it slightly greener than its neighbors.

Aerial photos provided by the Royal Commission on the Ancient and Historical Monuments of Wales show the cropmarks appearing over many well-known Welsh archeological sites.

Toby Driver/RCAHMW Cropmarks near Langstone, Newport, Wales.

Toby Driver/RCAHMW A Roman site near Magor, south Wales.

Toby Driver/RCAHMW A view of a Roman fort in Pennlwyn, Wales.

Toby Driver/RCAHMW Trewen Caerwent

Toby Driver/RCAHMW Cross Oak Hillfort, Welsh Iron Age site.