The pyramids are built up by sedimentary rocks dated from the Neogene and Quaternary periods. The sediments were deposited 5 to 3 million years ago when the graben valley surrounded by the mountain ranges of Pirin, Belasitsa, Ograzhden and Maleshevo used to be a lake. The lower Neogene layer is over 100 m deep and is composed by lightly soldered grey to yellowish sands and gravels made up mainly of quartz grains and clay strata. The 100 m deep Quaternary layer above forms most of the pyramids and is composed by lightly soldered or unsoldered rust and reddish conglomerate. That colour is due to the iron hydroxides produced by the decomposition of minerals.