In my previous article, I explained how early scientists set down many of the basic laws stratigraphers use to interpret rock sequences. Many of these dealt with interpreting how rocks are originally laid down. As postulated by Nicolaus Steno (1638-1686), sedimentary rocks must be laid down horizontally. As more sediments are laid down, the older rocks become buried by the younger rocks over them. With this knowledge, it is possible for a geologist to study a rock outcrop and before he or she even begins, they know a lot about the rocks in front of them.
However, nature usually conspires to never make it this easy. Many times the rocks are tilted, folded, or otherwise altered. Sometimes, there is a break in the rock record where no sediments were laid down, or the rock was eroded away. This break representing erosion or non-deposition can either be easy or difficult to see, depending on the types of rocks being observed. James Hutton (1726 – 1797) first described the evidence for this break in the rock record. He was impressed by a placed called Siccar Point in Scotland where some of the rocks were laid down horizontally, but the rocks underneath them were at an angle. The only explanation Hutton felt was plausible was that the underlying rocks had originally been laid down horizontally, then were tilted and eroded and more rocks were then deposited on top of them. Hutton realized that this scenario represented a large gap in time between the first set of rocks being laid down, then tilted, and the second set of rocks deposited. These gaps in the stratigraphic record are called unconformities. An unconformity can be traced along an outcrop and represents a break in the sequence in which deposition ends and erosion begins. These breaks can be relatively short in time, or can last for millions of years.
There are four different types of unconformities. Two of these types are easy to see. The first is the type that Hutton first saw at Siccar Point. This type is known as an angular unconformity. In this type of unconformity, the sequences of rocks beneath the unconformity are tilted with respect to the rock sequences above the unconformity. This type of unconformity usually represent a large gap in the geologic record (usually millions of years) since the tilting of the rocks and their subsequent erosion takes time.
