Conglog quarry was a small enterprise situated to the north-west of Tanygrisiau, near Blaenau Ffestiniog in Wales.

The land he leased covered 177 acres (72 ha), and was mainly to the north of the stream and the track that ran through the site to reach Rhosydd.

In 1865/6, they also built Tan-yr-Allt, a row of cottages close to the Conglog mill site, to house some of the Rhosydd workers and their families.

[1] In order to develop the quarry, Roberts sub-let part of the land to W H B Kempe and T M Matthews, who were from Devon.

They paid £900 plus £25 per year for the lease, with an additional £2 per acre for any land they used for tipping, and one-twelfth of the value of any slate which they produced.

The Conglog Slate and Slab Company Ltd was set up in 1873, with a working capital of £40,000, derived from 4,000 shares valued at £10 each.

[2] All was not well, as Matthews left the undertaking in 1873, and assigned his part of the lease to another shareholder called Edward Betteley, despite the fact that the company should have owned it by then.

The man in charge of this was Thomas Horswill from Tavistock in Devon, who had been the accountant and secretary since the formation of the company in 1873.

As Kempe had died, and Matthews was "insane", this presented a problem, but somehow he managed to resolve it, and sold the lease to two of the shareholders, Betteley and Gillow, for £850, the latter a doctor from Torquay.

1902 and 1903 were the most productive years for the quarry, and from 1903, when Ross surrendered his lease, Cadwaladr Roberts, one of the six quarrymen, acted as agent.

Rhosydd had attempted to do the same for many years, and had eventually resorted to the route down Cwm Croesor to the west, because Cwmorthin would not give their consent.

[9] Just below the terrace, the new Lake Incline would drop the level to that of Cwmothin's Cross Mill and their exit tramway.

[12] Operation of the Lake Incline was the responsibility of Conglog, although after 21 years, ownership would transfer to Cwmorthin, who would make a payment for the drum and the rails.

[13] The area surrounding Blaenau Ffestiniog contains five workable veins of Ordovician slate, which follow a semi-circular path around the northern edge of the town.

[4] According to a prospectus issued by the Conglog Slate and Slab Company in 1873, three of them were accessible from the quarry, although there is no evidence that the third was ever worked or even found.

A plan produced in 1903 showed the adit running on to the north vein, and the beginnings of a chamber there, but this was not confirmed by a survey carried out in the 1980s.

[19] Water was brought down the hillside in leats fed by several streams, and there were two sections of raised aqueduct or launder.