Calgary Bay Dykes and Sills


Calgary Bay in north west Mull is a great place to see igneous dykes and sills cutting through the Palaeocene lavas. The intrusions are right beside the track that runs along the shore of the bay and are very accessible.


Calgary beach looking to the north shore. Palaeocene lava flows make up the high ground behind the beach

The Pier Dyke. There is a very prominent dyke of the Mull Swarm near the old pier. The pier itself looks like it has been made from Ross of Mull Granite, its pink colour in stark contrast to the basaltic rocks.

From the old pier looking to the famous dyke running down the hillside.

View of the dyke when approaching the pier. The dyke has been used as part of the walls for the old boat-house.

Dykes are common in Mull, in particular those that belong to the so-called "Mull Swarm". They are found throughout the island, but this one at Calgary is very much a text-book example. Another very similar looking dyke is seen on the south shore of the Bay. This may well be a continuation of the dyke at the pier. It is clearly visible on the shore below a house. See the pictures below.

The Shore Dyke Other dykes occur on the shore. This one is not as impressive as the Pier dyke but is still a good example of a typical mull Swarm dyke.

Another Palaeocene dyke of the Mull Swarm, vertical to sub-vertical, runs in a NW-SE direction.

This particular dyke shows good polygonal jointing, the columns lying horizontally, at right angles to the cooling surfaces:

Knobbly polygonal jointing in the dyke on the shore


In addition to the dykes, there is also a very good example of a sill to be seen at Calgary. It can be found on the shore but is best exposed just above the track. For a diagram explaining the difference between a dyke and a sill, try this link


Sill (grey colour), underlying lavas  (brown colour)above. near the path to Calgary pier.


The sill has been intruded into the lavas - at the location only the top Field Trips is visible. A "chilled margin" is really obvious showing the development of tachylite, a very fine grained, black rock, almost tar-like in its appearance. Pictures below show this in more detail. The sill, like the dyke on the shore, has been sampled - geologists' core holes are clearly visible, and some would argue rather unsightly.


Other photos of the location:

Click on an image to open a larger picture.

The dyke on the shore and Calgary Bay, looking south
Shore dyke and bay
The dyke on the shore

The dyke on the shore
The dyke on the shore, looking south

Shore Dyke
Side view of the dyke showing jointing

Side view of the Dyke
Holes in the dyke made by geologists taking cores

Core holes in the Shore Dyke
Continuation of the Pier Dyke on the other side of the bay?

Pier dyke on the other side of the Bay?
Close up view of dyke on south side of bay

Close up view of the dyke on the south
side of the bay
The dyke near the pier

Pier Dyke
The dyke seen from the old pier

Pier Dyke seen from Pier
Dyke has been used in the construction of the boat house

Old boat-house and Pier Dyke
Sill, under lava flow

Sill, under lava flow
Another view of the sill

Another view of the sill
Close up view of the sill showing tachylite at the margin

Close up of sill, showing tachylite

Similar or related Geological Sites:

Dykes are found all over Mull, the ones belonging to the Palaeocene "Mull Dyke Swarm" being the commonest. They have their greatest concentration along the shores of Loch na Keal and in the Croggan area. Good examples include.


  1. Ardmore Point Dykes
  2. Croggan Dykes
  3. Loch na Keal Dykes
  4. Lagganulva Ash Bed (dykes cut through these rocks)



Bailey, E.B., Clough, C.T., Wright, W.B. et al. (1924) Tertiary and Post-Tertiary Geology of Mull, Loch Aline and Oban. Memoir of the Geological Survey of Great Britain, HMSO, Edinburgh.