Not every anomaly discredits a theory

In 1832, Henry Thomas De la Beche undertook a geological survey of Devon. This work eventually led to the establishment of the Geological Survey in 1835, but not before De la Beche had suffered great public embarrassment. While he mapped the strata in Devon, his colleagues Roderick Murchison and Adam Sedgwick were similarly engaged in Wales, respectively establishing the Silurian and Cambrian systems. Everyone assumed that the rocks in Devon were around the same age as those in Wales. But in 1834, when De la Beche found Carboniferous fossils in coals embedded in the middle of rocks he thought were Silurian, he declared that William Smith’s theory of ordering the rocks using the fossils they contained must be wrong. Murchison leapt to the attack even though he had never examined the Devonshire strata himself. This deeply offended De la Beche, who believed that the validity of field work should not be questioned on theoretical grounds.

When Murchison and Sedgwick examined the Devonshire rocks themselves in 1836, they found that De la Beche had made a mapping error. The coal deposits were in fact at the top of the Devonshire strata rather than in the middle, so they assumed the coal must lie at the bottom of the Carboniferous, sitting on the older Silurian rocks – what’s known as an unconformity. De la Beche was publicly criticised and the fledgling Geological Survey was nearly brought to an untimely end. But the arguments didn’t end there. De la Beche was forced to admit his error, but he insisted that there was no unconformity between the coal strata and the older Silurian rocks. To their discomfort, Murchison and Sedgwick could not identify an unconformity either and had to admit that there wasn’t one.

There followed much debate and extensive investigations ranging as far afield as Russia, where in 1840 Murchison discovered a layer similar to the coal found in Devon, positioned between well-defined Silurian and Carboniferous deposits. This finally put an end to what became known as the Great Devonian Controversy and led to the definition of a new period called the Devonian. It also led to a fundamental change in geological practice and the value of fossils as stratigraphic indicators was established beyond question. Fortunately, the Geological Survey survived this early trauma and De la Beche became its first Director. Both the Geological Society and the Geological Survey still exist today.

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