A while back we wrote about one of the founding fathers of Geology, eighteenth century Scottish scientist, James Hutton. The concepts that Hutton developed, of deep time and the underlying constancy of physical laws and processes, came from careful observations of the outcrops around him. Some of the most famous are Hutton's 'unconformities'. At a number of places across Scotland, Hutton identified that the rock record revealed gaps in time, with much older rocks directly overlain by younger rocks that must have been deposited many millions of years later. This demonstrated to him the scale of geological time, and how it can be identified in the geological landscape.
The key to the gap in time came from the geometric relations that Hutton observed. The clearest example is at Siccar Point, on the east coast of Scotland, near Edinburgh. Here, Devonian sandstones, deposited 345 million years ago on the arid landscape of the Euramerican supercontinent lie in almost horizontal beds, dipping shallowly out to the current-day sea. But the rocks directly beneath lie with their bedding near vertical. They were deposited on the floor of a Silurian ocean, 425 million years ago, and must have been tilted through 90˚ since they were deposited. A chunk of time, amounting to around 80 million years, is missing from the rock record. Hutton recognised (qualitatively) that gap and the order of events from the relative geometries of the beds. The Silurian marine sediments had lithified, were then uplifted and tilted, and turned on end. Although he could not say much about the gap in time, it follows that erosion formed a land surface, on which the later Devonian sandstones were deposited. The boundary between the basal conglomerate and the Silurian sediments is the unconformity recognised by Hutton.
James Hutton had recorded similar relations in different rocks a year earlier, in 1787, at Newton Point on the island of Arran, west Scotland. Here once more shallowly dipping red sandstones, formed in an arid terrestrial environment, lie on top of steeply-dipping and cleaved grey rocks. The underlying rocks here go by the name of Dalradian, after that of an ancient native tribe. Low-grade metasediments, they were deposited from a continental shelf into a late Precambrian ocean and have subsequently undergone regional scale deformation. The overlying red sandstone conglomerate belongs to the late Devonian and early Carboniferous. Indeed, at Newton Point geologists have subsequently identified a Devonian unconformity, the land surface in the late Devonian formed from tilted and eroded Precambrian basement, and a further disconformity. This represents a gap in time, but with no erosive-tectonic disturbance in the sedimentary sequence, before the subsequent lower Carboniferous conglomerates were deposited.
We take students to Newton Point every spring, to discover for themselves the first steps in reading the rocks. Here is part of this year's group, enjoying Hutton's unconformity at the end of a long day in the field.
Image: Newton Point, first year field trip.
