See study by E. B. Bailey (1967).
In it, he outlined his theory that the "solid parts of the present land appear in general, to have been composed of the productions of the sea, and of other materials similar to those now found upon the shores. From this he deduced that the land was a composition which had been formed by the operation of second causes in an earlier world composed of sea and land, with tides, currents, and "such operations at the bottom of the sea as now take place" so that "while the present land was forming at the bottom of the ocean, the former land maintained plants and animals; at least the sea was than inhabited by animals, in a similar manner as it is at present", and that most, if not all, of the land had been produced by natural operations involving the consolidation of masses of loose materials collected at the bottom of the sea, followed by the elevation of the consolidated masses to their present position.
Angular unconformities had been noted by earlier geologists who interpreted them in terms of Neptunism as "primary formations", so Hutton wanted to examine such formations himself to look for support for his theory of Plutonism. On a trip to Arran in 1787 he found his first example of an unconformity, but the limited view did not give the information he needed. It occurs where some vertically oriented Dalradian schists are overlain by more recent (possibly Carboniferous) horizontal sandstone. The difference in dip between the two rock layers is now particularly obvious when at the site.
Later in 1787 Hutton noted what is now known as the Hutton Unconformity at Inchbonny, Jedburgh, in layers of sedimentary rock. He later wrote of how he "rejoiced at my good fortune in stumbling upon an object so interesting in the natural history of the earth, and which I had been long looking for in vain." That year, he found the same sequence in Teviotdale.
In the Spring of 1788 he set off with John Playfair to the Berwickshire coast and found more examples of this sequence in the valleys of the Tour and Pease Burns near Cockburnspath. They then took a boat trip from Dunglass Burn east along the coast with the geologist Sir James Hall of Dunglass. They found the sequence in the cliff below St. Helens, then just to the east at Siccar Point found what Hutton called "a beautiful picture of this junction washed bare by the sea". Continuing along the coast, they made more discoveries including sections of the vertical beds showing strong ripple marks which gave Hutton "great satisfaction" as a confirmation of his supposition that these bed had been laid horizontally in water.