Alexandra Walsham's latest book is much like the landscape it describes: complex and many-layered, ranging over wide vistas, gentle scenery and striking landmarks. Her subject is the religious landscape of Britain and Ireland in the early modern period (from the 15th to the 18th century) and, more importantly, the ways in which that landscape was perceived, shaped, altered, attacked and defended in the religious imagination of the time. It is a study in the mutability of both spiritual topography and historical memory.

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Historical debates about the Reformation were once conducted largely in terms of intellectual formulations and the individuals who upheld them. When the debate escaped the confines of theology, it became a series of discussions about religion in both politics and society. More recently, an awareness of material culture has underlined the importance of ritual objects and sacred space. Never before, however, has this debate extended to include rocks, trees, wells, rivers and mountains. Walsham has uncovered an extraordinary assortment of sacred sites, and has carefully teased out of some rich but problematic sources just how these features of the landscape fitted into religious belief and practice before, during and after the Reformation.

We might have assumed that the arrival of Protestantism would destroy the sacred associations of the British landscape, and certainly many zealous Protestants worked hard to purge their realm of superstitious remnants, rubbishing popular beliefs about sacred features in the natural world. Yet Walsham shows that other equally zealous Protestants sought refuge in the landscape, preserving elements of pre-Reformation legend and even promoting the "resacralization" of both old and new features. The legacies of the Reformation were far from being merely negative; they also lent themselves to "creative directions".

Far more than mere survivalism, this was the reconfiguration of beliefs about the sacred in line with Protestant priorities - the symbolic tending to replace the sacramental. The consequences of this could be highly convoluted, as with the holy wells that saw Protestants and Catholics simultaneously accepting divine help, differently comprehended. They could be faintly absurd: for instance, legends about the giants referred to in the Bible survived far better than those about goblins, which happen to be absent from Scripture. Nor did miraculous features have to draw on pre-Reformation inspiration: many were freshly minted in the 17th or even 18th centuries, like the Shropshire well that became holy once John Wesley had drunk from it.

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The reformation of the landscape was a far more intricate and surprising process than we might think. If it involved destruction, it also embraced the concept of the natural world as a providential metaphor, and earthquakes, floods - even "monstrous cucumbers" - as divine messages. In this book, Walsham deals swift and decisive blows to several common assumptions, one of them the notion that Protestantism hastened secularisation. The Protestant landscape could prove just as miraculous and magical as its pre-Reformation Catholic equivalent, but its magic ran through slightly different channels.

Equally, notions of Enlightenment are brought under precise and thoughtful scrutiny, as Walsham shows that there was just as much an alliance as a conflict between theology and science in the approach to the natural world. The advance of rational explanation was matched by a persistent view that the Earth was a living record of biblical events and divine intervention. A forestry expert in 1670 could still date trees back to the time of the Flood, a few even to the Creation. Meanwhile, the link between piety and medicine was not severed so much as recalibrated, since the transition from the healing wells of the pre-Reformation era to the fashionable spas of the 18th century maintained the underlying conviction that the healing waters were a blessing from God.

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Different perceptions could live in a state of mutual toleration. The 17th-century entrepreneur pushing the concept of a Yorkshire spa was giving a new spin to holy wells still venerated by recusant Catholics. If Walsham's Reformation is capacious enough to include several different strains of Protestant thought, it also has room for an understanding of early modern Catholicism that slices neatly through the usual idea of a pre-Reformation survivalism at odds with a newly militant Counter-Reformation.

Walsham suggests instead that traditional and reformed Catholics actually worked in harmony: the agents of Tridentine reform were happy to engage in "constructive dialogue" with the old beliefs and customs they encountered. But constructive coexistence went further than this where a Protestant regime could release Catholic recusants from prison to convalesce at Buxton or Bath. New miraculous legends could emerge from Protestant landmarks: at Lutterworth in Leicestershire, a holy well marked the site where John Wycliffe's bones had been burned.

Some transactions were more straightforward: the holy well in County Dublin where Oliver Cromwell washed his feet dried up until restored by Catholic prayers. But elsewhere, sanctity was transferable: the well near Bristol that had once commemorated the Virgin Mary was, by the end of the 17th century, deriving its healing powers from the tears of a local woman widowed in the Civil War. In other places, the holiness fell away, leaving only the merrymaking behind, or on the Isle of Lismore in Scotland, the horse racing that was a relic of a former religious ceremony to bless animals. The pluralism that was the "unwanted but unavoidable" legacy of Reformation left its mark upon the perceptions of the surrounding natural world in subtle and often unexpected ways.

Walsham is working on such a large canvas it is possible that some may take issue with small parts of the picture. Her reservations about the late medieval Church, seeing the veneration of holy wells and pilgrimage sites as a "distraction" from parish worship, could be challenged by those who see pre-Reformation piety as integrated despite its variation. Amid the wealth of evidence, some may question the connections between different phenomena, or challenge the strength of the trends that Walsham identifies.

One of the obvious strong points of the book, its inclusion of Scotland and Ireland, may also leave it open to criticism about the coherence of the overall picture. It could be argued, for example, that Catholic survival was a different phenomenon on the west coast of Ireland than it was in the southeast of England. On the whole, however, it is hard to contrive even small objections to a work so assured in its scholarship, at once so sensitive in its sifting of the evidence and also so original in its conclusions.

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The overall picture is vivid, astoundingly detailed and deeply compelling in its conceptual range and its forthright analysis. This book moves with both grace and authority over a vast tract of time and space, giving a whole new dimension to the Reformation debate, and contributing to several other related discussions as it goes. It moves our understanding of religious change on to a different level: where the layout of a garden could demonstrate a belief in divine right kingship; where a holy well could go on predicting into the Victorian era, prophesying the First World War; where martyrdom was applied to architecture, and Protestants such as John Calvin found in the natural world God's "most beautiful theatre".

Walsham's main achievement here might indeed be to "soften and complicate the picture of annihilation and rupture", but she does much more besides. Charting the topography of religious conviction and the panorama of magic and memory, she has reconfigured a landscape of her own, contributing an outstanding landmark to the scholarly terrain.

The Author

As a child, Alexandra Walsham wanted to be a lighthouse keeper's daughter. But she distinguished herself in a different way in September 2010 by becoming the first woman to hold the post of professor of modern history at the University of Cambridge.

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She was born in Cornwall to geologist parents, and her interest in history is something of an aberration in a family of scientists. Aged five, she became a "ten-pound Pom" when the family emigrated to Australia.

Walsham received a bachelor's degree in history with English literature from the University of Melbourne in 1987 then spent a year travelling Europe and Africa before gaining a master's in history at Melbourne. She won a Commonwealth Scholarship to Cambridge to pursue a PhD in early modern British religious history.

In 1996, she was appointed lecturer at the University of Exeter, where she would spend 14 years, latterly as head of the history department. Her book Providence in Early Modern England won the 1999 Longman-History Today Book of the Year prize, and in 2009 she was elected a Fellow of the British Academy.

Now living in the flat fenlands of East Anglia, Walsham says she misses both sunny and relaxed Australia and the rolling hills of Devon.

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