Over the years several geologists and geophysicists from the British Antarctic Survey have been employed to study the Marguerite Bay area and the following description of the rock types is a simplified version of BAS Geomap series “Geological Map Sheet 3 : Adelaide Island to Foyn Coast” published by NERC in 1994.
The bulk of the rocks exposed today on the mainland of South Graham Land are plutonic & volcanic rocks from the zone of molten magma that continues northwards as the Scotia Arc. This is where the South American continental plate is slowly moving south eastwards and being driven down (subducted) under the Antarctic continental plate, which is being consumed at the collision zone. Plutonic rocks are those which have crystallized from magma cooling slowly beneath the surface of the Earth while volcanic rocks are formed from magma which has erupted from a volcano and therefore cool and crystallize faster.
The rocks can be dated by measuring the proportions of different isotopes of Rubidium & Strontium or of Potassium & Argon. Such measurements indicate that most of the rocks in Marguerite Bay are around 180 Ma (million years) old, although the range is from late Triassic (209 Ma) to early Tertiary (60 Ma).
Solid colour in map indicates observable rock; pale colour indicates presumed identity of snow & ice covered ground
As the very detailed map above shows, Horseshoe Island, like the nearby continent & islands, is mostly made of Granite (the pink areas in the map). Granite is a plutonic rock consisting of large or small crystals of quartz, alkali feldspar & plagioclase (in various proportions), usually with some hornblende or biotite. Pure quartz crystals are colourless and transparent (clear) or translucent. Most feldspar crystals are white, but they can be coloured. If they are pale and show striations or are dark grey or black then they are ‘plagioclase’ and if they are uniform white, pink or blue they are generally ‘orthoclase’. Hornblende consists of black, dark brown or greenish crystals, made up of silicates of calcium, magnesium, iron & aluminium, and biotite is similar but is micaceous and, like other micas, has a layered structure which means that it readily cleaves to produce thin sheets or flakes. 
However, most tourists will land on the beach near to the historic hut and the area to the north of this is mostly Gabbro, a coarse-grained and usually dark igneous rock, shown as green areas on the map. Gabbro is dark because it is mostly composed of calcium rich plagioclase, iron oxides and pyroxene with usually less than 5% quartz. Sometimes quartz or olivine crystals make or up considerably more than 5% in which case the rock is classed as Quartz Gabbro or Olivine Gabbro. Pyroxene & olivine crystals tend to be pale to dark green in colour. There are also areas of Diorite ,a speckled, coarse-grained igneous rock which results from the partial melting of granite or gabbro above a subduction zone. The photo below shows some gabbro inside a mass of diorite
Metamorphic rocks, mostly gneisses (purple of the map) can also be found on Horseshoe Island. These frequently have a banded appearance as in the photograph below.
as well as some rocks of Volcanic origin (blue on the map), mostly basaltic lavas and pyroclastic rocks, in other parts of the island. Basalt is a dark, fine-grained igneous rock and is the most common type of lava. It sometimes shows columnar jointing, which indicates rapid cooling and sometimes has vesicles where gas bubbles where trapped. It makes up most of the ocean floor. 
Ash, pumice & obsidian are examples of pyroclastic material, fragments of rock erupted explosively from a volcano.
You may also come across Dykes, predominantly basaltic in nature but of different colours, that cut across outcrops of rock since these are common throughout Southern Graham Land. Some of these intrusions are of highly distinctive pale rock with lots of quartz & pink or white alkaline feldspar making up over 50% of the rock (Felsic dykes) while others (Mafic dykes) are much darker with up to 30% plagioclase and pyroxene, and show a distinct colour contrast.
Potassium - Argon dating suggests most of these dykes were intruded around 88 Ma ago and their composition suggests they are made of magma produced at a consuming plate margin, i.e.are derived from the Antarctic continental plate.
The northern Marguerite Bay has a greater gravitational influence than average (a positive anomaly) which suggests that a large dense batholith underlies the crust in this region. Minerals, such as zinc, copper, lead & tin, contained in this batholith sometimes leach out and emerge onto the surface at cracks, giving the appearance of coloured veins. Some of these are very conspicuous near the hut.
No fossils have been found to date on Horseshoe Island but leaves and fossil wood of cycads & conifers have been found on nearby Lagotellerie Island. On Adelaide Island away to the west fossil ammonites & molluscs (bivalves and oysters) of many species have been found while fossilised tree trunks, up to 4 m long and 50 cm across, are present on the Dion Islands so if you think you spot any kind of fossil make sure you tell a tour guide or geologist!
TASKS & QUESTIONS
1) From the description and images in the listing try to identify what type of rock surrounds the summit cairn at the listing co-ordinates and say what makes you think your answer is correct? Try to say more than just "igneous" or "metamorphic".
2) As you descend towards the hut, about 30m SE of the summit you hit a band of very different rock which has weathered differently. Describe its colour and pattern of weathering.
3) Lower down, near the hut are masses of rock in which dark crystals are particularly large and prominent. What rock type do you think this is and which mineral in it has the largest crystals?
4) There is a coloured mineral very obvious in the rocks close to the hut at S67 48.495 W6717.682. Tell me what colour it is, how it seems distributed and what is the mineral this is likely to be?
5) Finally briefly explain what a batholith is.