Darwin summarised his speculation in his essay on the "Elevation of Patagonia". Though tentative, it challenged Lyell's ideas. Darwin drew on measurements by "Beagle"'s officers, as well as his own measurements, to propose that the plains had been raised in successive stages by forces acting over a wide area, rather than smaller-scale actions in a continuous movement. However, he supported Lyell in finding evidence to dismiss a sudden deluge when normal processes were suddenly speeded. Seashells he had found far inland still showing their colour suggested to him that the process had been relatively recent and could have affected human history.