Figure 15. The keystone rolling to its final position a) the outlet wall of MB-A b) the inlet wall of MB-B

From

The World’s Oldest Bridges - Mycenaean Bridges

Karas Slawomir, Nien-Tsu Tuan

American Journal of Civil Engineering and Architecture. 2017, 5(6), 237-244 doi:10.12691/ajcea-5-6-3