Figure 2.
James Hutton’s unconformity at Siccar Point, Scotland—the contact between the gently inclined Devonian Old Red Sandstone and vertically dipping Silurian graywacke that established a compelling case for the vast scope of geologic time. The expanse of time required to uplift and erode the two mountain ranges that were the source for the sand in these deposits was unimaginable to Hutton. Photo by David R. Montgomery.