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.
