• During the Tertiary Period Arran was at a latitude of 51-54^oN. During this time a lot of plate activity took place and the opening of the North Atlantic occurred. Arran lay in an area where a lot of volcanic activity took place, this activity was in part related to major plate movements. It is during the tertiary that Arran began to take on a shape similar to it's present form. There are some excellent texts on the Tertiary rocks and geology of Scotland and Arran and readers interested in a more detailed discussion of the tertiary Period should refer to M^cKerrow and Atkins 1989, MacDonald and Herriot 1983 and  Sutherland 1982 and references there in.  Our simplified approach to the Tertiary of Arran is to break it into several generalised events and related units (a-e), based on the more detailed proposed Tertiary Chronolgy of M^cKerrow and Atkins 1989.  The division of the Tertiary volcanic rocks of Arran and the surrounding Isles is based on geochemical analyses of rocks for elemental and isotopic ratios coupled with field observation of the Tertiary Rocks in situ (McKerrow and Atkins, 1989, page 26, and references there in).

a.) During most of the Tertiary except the very last part, basaltic dykes (with a range in compositions) were intruded into country rocks Image 1. In some places on Arran in particular around the coast at Kildonan, there are so many dykes to see, that they are called dyke "swarms" Image 2.

Image 1. A Tertiary igneous dyke, cross-cutting Permian wind blown sediments at Corrie shore.

Image 2. A tertiary dyke on the shore at Kildonan with the Isle of Pladda in the background.

b.) The next simplified sub-division of the Arran tertiary events/rocks, is thought to have  occurred early in the Tertiary Period (ca. 63-60 million years ago), and involved the production of basalt plateau lavas, erupted through fissures. The only remnants of these lavas can be seen in the collapsed caldera of the  Tertiary Central Ring Complex of Arran (see d below).

c.) The next Tertiary sub-division is the Northern Arran Granite unit/event  (ca. 60.3-58.8 million years ago). A massive granite pluton forced it's way up through the north of Arran, forming the northern Arran mountains (Image 3) and tilting and locally metamorphosing the Dalradian schists. The granite has a coarse grained inner portion and a finer grained outer portion. Associated with the northern granites are aplitic rocks and pitchstones intrusives.

Image 3. A view of some of the Granite Mountains of North Arran taken from Goat Fell.

d.) The fourth sub division of the Tertiary of Arran, is that of the Central Ring Complex, with ages at ca. 58.3 million years plus or minus 4.4 million years.  During this time in the central area of Arran a volcano existed, the caldera of the volcano collapsed on itself and and there were other minor volcanoes within resurgent caldera.  Many volcanic rock types were produced including granites, gabbros, felsites, basalts, andesites, pyroclastics and other volcanic rocks. During the collapse of the volcano of the Central Ring Complex, blocks of Jurassic and Cretaceous sediments fell into the collapsed caldera and are now the only remains of those sediments on Arran.

e.) The fifth general sub division of the Tertiary of Arran, can be said to have taken place at around 58.5 million years ago plus or minus 1.6 million years. during this time in the Tertiary the Ails Craig plug was formed, it is composed of a riebeckite granite, the Holy Isle Sill was formed Image 4 , the stunning Drumadoon Sill was formed Image 5, and the sill that the Isle of Pladda is partly composed of formed. numerous other acidic and basic sills and dykes were formed and a basic cone sheet formed as well.

Image 4. A view of the Holy Isle from Arran, the main rock forming the Holy Isle is a Tertiary sill, composed of a riebeckite trachyte (akin to granite).

Image 5. A view of the Drumadoon Sill, which displays beautiful columnar jointing and is composed of quartz-feldspar porphyry.

• During the latter part of the Tertiary period on Arran, a lot of the Central Ring Complex was uplifted and eroded, erosion of the northern granite mountains also occurred.