Dorothea opened in 1820 and remained in production until 1970. It is in the parish of Llandwrog, close to Talysarn, Caernarfonshire. The land the quarry stands on was owned by a Richard Garnons (1774 -1841).
Dorothea Quarry was bought in 1835 by an Englishman called Muskett. He spent heavily on new equipment to raise the waggons from the quarry bottom. But he overspent, and was declared bankrupt a few years later. The quarry was closed with three months wages owing to most of the workers. They consequently revolted and demolished the new house Muskett had built for himself - Plas y Cilgwyn.
By the 1840's production at Dorothea had built up to about 5,000 tonnes per annum and had reached over 17,000 tonnes by the 1870's. The future looked good for Dorothea but serious flooding problems then befell the quarry. In 1884 several men were drowned when the pit was engulfed. In 1895 the Afon Llyfni which flowed through the valley was realigned and deepened to flow to the south of the slate workings. This cured the flooding problems to some extent but as the workings deepened, the need to continually pump out water became a constant drain on the quarry's profits. In 1904 the decision was taken to install a Cornish Beam Engine on site to replace the waterwheels.
The main driving force for quarrying in the valley was a Lancastrian - William Turner (1776 -1857). The original name for the quarry was Cloddfa Turner but it was renamed Dorothea. The workings grew out of a series of smaller workings with names such as Hen Dwll, Twll Bach, Twll y Weirglodd, Twll Coch and Twll Fire. Over the years these pits were deepened and amalgamated. Dorothea Quarry was worked between 1829 and 1849 by William Turner and his son-in-law John Morgans. Turner and Morgans had worked it to great advantage but they gave it up in April, 1849 as it had not latterly proved remunerative.
Dorothea Slate Company floated in April, 1849. A number of Nantlle quarrymen who had been working the small PwIl-y-fanog quarry in the valley formed the idea of floating a company to buy and work the larger Dorothea quarry. Through the co-operation and the influence of the Rev. John Jones, Talysarn, the famous Welsh preacher, sufficient capital was raised to form a company. A hundred shares of £25 each were soon taken up and the lease, machinery and fixed plant of the quarry were bought for £3,000, half the purchase money to be advanced immediately and the other half within a year. The Rev. John Jones, as Chairman of the Company, presided over the meetings of shareholders which were held every two months at the quarry office.
The minutes of these meetings were kept in Welsh and throw a light on the democratic government of the concern during its early years when the quarrymen shareholders had a voice in its management and in determining general policy. It has been said that to mix religion and business is to spoil two good things; the Rev. John Jones would not agree with that dictum because we find him making use of his position as Company Chairman to bring economic pressure to bear on some of the quarrymen in an effort to make them give up their drunken habits.
The continual need for raising new capital meant that the quarrymen were very soon holding less than half the shares and they had to give up the idea of controlling the concern and. sold 20 shares to Mr. Williams of Plas y Blaenau, Denbighshire, one of the shareholders, for £2,000 or four times their original value. The quarry did not prosper for some length of time, and the Rev. John Jones, who had himself been a practical quarryman and was anxious lest his friends should lose their money, in 1851 himself became quarry agent at a weekly wage of 25/-. In the following year the selling of the quarry was discussed but at a meeting of the shareholders held on September 20th, 1852, those who opposed the selling of the quarry carried the day because they held a majority of four shares over the party which was in favour of selling. In a few years the concern became very profitable. Mr. Williams gradually bought up more and more shares so that soon he had a controlling interest, and this family has ever since held the majority of shares in the concern.
In 1828 the Nantlle Railway opened giving the quarries of the valley a route to the sea. The horse powered railway was of 3' 6" (105cm) gauge and ran originally to Caernarfon. From 1872 the tramway ran only as far as Talysarn where connection was made with the national rail network. Over its lifetime the route of the railway was moved many times as the quarries expanded. Much of its route is traceable today as far as the easterly terminus at Penyrorsedd Quarry. Dorothea Quarry used the Nantlle Railway to despatch slate from 1829 until 1959.
Following the steady decline in the slate industry in Wales, the Dorothea Slate Quarry Co. Ltd entered liquidation in 1970, and the quarry was closed in 1971.