Don’t let the strange title put you off. Ammonite is a whorl – a shell fossil, commonly found on the sea shore of Lyme Regis in 1840s Dorset. The main character in this film is based on a real woman, Mary Anning, a brilliant palaeontologist who spent her days searching for rocks and cracking them open to find the treasures within.

That may not sound too riveting, but riveting is the right word to describe Ammonite, which stars Kate Winslet as Anning and Saoirse Ronan as Charlotte Murchison, the younger woman who becomes her fossicking partner and lover.

Ammonite is not a bio pic. There is no evidence that Mary Anning and her real-life friend Charlotte Murchison had a raging love affair, and many critics are irritated that the film does not stick to the known facts about her life. However, this misses the point; Ammonite is a fictional drama, a love story about the passion between two women.

Even if the pair were lovers, no one would have spoken openly about it in those days. When laws against homosexuality were first passed in England, they did not include women; Queen Victoria simply refused to believe that they would behave in such a manner.

British writer-director Francis Lee (God’s Own Country) uses a very understated style to tell his story of these two women whose lives become so intertwined. The film is much like a symphony which begins very softly and slowly and builds gradually towards its explosive climax.

It takes a while to settle into the low tempo. Mary lives with her ailing mother in a cold, cramped house where they make fossil trinkets to sell to tourists and eke out a meagre living. She is a closed-off, emotionally repressed woman who rarely speaks and spends her days trudging along the bleak coastline searching for her precious fossils.

Sparse dialogue is used to convey the interior lives of both Mary and Charlotte, and therein lies the brilliance of Lee’s direction and the actors’ performances. The meaning of the film lies not in what is spoken, but in what is unspoken. The objects inside Mary’s home speak for themselves.

Winslet makes viewers feel Mary’s emotional bruises, despite her growing attraction for the younger Charlotte, whose palaeontologist husband – unable to cope with his wife’s melancholia – has paid Mary to look after her.

Ammonite is beautifully filmed, despite its bleak canvas. When the women crack open a huge fossil Charlotte finds on the beach, what follows is their own physical and emotional release.  As for the sex scenes, even though neither Winslet or Ronan is gay, Winslet has been quoted as saying: “Women know what women want, let’s not beat about the bush. Saoirse and I knew we could unite in our feelings about these characters and where they’re at, and to tell that story physically. It was a very joyful experience.”

The final shot in Ammonite is of the two women standing on opposite sides of a glass display case of Mary’s amazing discoveries in the British Museum, staring each other down. We are left to make our own conclusion about how it will all end.

At 45, Winslet is at her peak. Lee’s Ammonite is very different from anything else she has done, as his is truly an original voice and style. She literally disappears into the character of Mary Anning and we feel her frustration, her insecurities and, most of all, her loneliness, as if they are our own. Her performance is Oscar material.

Dr Susan Mitchell is an author and a theatre, book and film critic.