The rocks of Hart Side and Green Side are all part of the Borrowdale Volcanic Group, formed on the margin of an ancient continent during a period of intense volcanic activity, roughly 450 million years ago in the Ordovician Period.

Virtually all the rocks on the fell belong to the Birker Fell Andesite Formation.
This was formed predominantly by eruptions of mobile andesitic lava from shallow-sided volcanoes.
These rocks are part of a thick succession of lava sheets found around the western and northern sides of the Lake District.

Some microgranite dykes were later intruded into the andesite which now forms the crags of Glencoyne Head.
These are of early Devonian age and were probably associated with the later stages of the emplacement of the granite batholith which underlies the Lake District.

Also associated with the granite batholith was the creation of mineral veins in parts of the Lake District.
The richest known lead vein of all was found crossing the south-east shoulder of Green Side.
This, the Greenside Lead Vein runs in a north–south direction and dips steeply to the east at 70°.
It was up to 59 feet (18 m) wide in the highest workings of the Greenside Mine, but averaged about 8 feet (2.5 m) wide in deeper workings.
The vein contained galena in a mixture of quartz and smashed rock.
Galena (lead sulphide) is an ore of lead.
The ore from Greenside also yielded 10 oz of silver per ton of lead.
Higher levels of the vein also contained quantities of baryte (barium sulphate), although at the time this had no commercial value.
Very small amounts of blende (zinc sulphide) and chalcopyrite (copper-iron sulphide) were found in the deeper levels, but never in recoverable quantities.

The Greenside Mine worked this vein from some time in the 1700s until reserves were exhausted in 1962.
The main entrance was in Glenridding at Lucy's Tongue, but an exploratory level was driven into the cliffs at the top of Glencoyne, from just below the Miners' Balcony Path.
In 1955 this was incorporated into an emergency back exit from the mine.
Alfred Wainwright found the hole, but thought it was just a cave.

Underlying the andesite lavas of the Borrowdale Volcanic Group are the mudstones of the Skiddaw Group of rocks.
These were encountered in the lowest workings of the Greenside Mine, some 3,000 feet (914 m) beneath the surface of the fell.
The lead vein the miners were following became barren on encountering these rocks.
The rocks of Hart Side and Green Side are all part of the Borrowdale Volcanic Group, formed on the margin of an ancient continent during a period of intense volcanic activity, roughly 450 million years ago in the Ordovician Period.
Virtually all the rocks on the fell belong to the Birker Fell Andesite Formation. This was formed predominantly by eruptions of mobile andesitic lava from shallow-sided volcanoes. These rocks are part of a thick succession of lava sheets found around the western and northern sides of the Lake District.
Some microgranite dykes were later intruded into the andesite which now forms the crags of Glencoyne Head. These are of early Devonian age and were probably associated with the later stages of the emplacement of the granite batholith which underlies the Lake District.
Also associated with the granite batholith was the creation of mineral veins in parts of the Lake District. The richest known lead vein of all was found crossing the south-east shoulder of Green Side. This, the Greenside Lead Vein runs in a north–south direction and dips steeply to the east at 70°. It was up to 59 feet (18 m) wide in the highest workings of the Greenside Mine, but averaged about 8 feet (2.5 m) wide in deeper workings. The vein contained galena in a mixture of quartz and smashed rock. Galena (lead sulphide) is an ore of lead. The ore from Greenside also yielded 10 oz of silver per ton of lead. Higher levels of the vein also contained quantities of baryte (barium sulphate), although at the time this had no commercial value. Very small amounts of blende (zinc sulphide) and chalcopyrite (copper-iron sulphide) were found in the deeper levels, but never in recoverable quantities.
The Greenside Mine worked this vein from some time in the 1700s until reserves were exhausted in 1962. The main entrance was in Glenridding at Lucy's Tongue, but an exploratory level was driven into the cliffs at the top of Glencoyne, from just below the Miners' Balcony Path. In 1955 this was incorporated into an emergency back exit from the mine. Alfred Wainwright found the hole, but thought it was just a cave.
Underlying the andesite lavas of the Borrowdale Volcanic Group are the mudstones of the Skiddaw Group of rocks. These were encountered in the lowest workings of the Greenside Mine, some 3,000 feet (914 m) beneath the surface of the fell. The lead vein the miners were following became barren on encountering these rocks.
Hart Side