The natural history of Bute begins with the story of its rocks, and this told in the Museum with samples of rock from every part of the island.
Bute is cut by three major valleys resulting from ice action, and is thus divided into four sections, each with its distinctive qualities. The northernmost part has extensive moorland, and many low but sometimes craggy hills, reaching 278m at Windy Hill, the highest point of the island. There is an area of conifer forest, and several older woods, mainly of oak and birch. The rock here is schist (as in Cowal), a hard, grey metamorphic rock, with strata often steeply tilted and contorted. There is no road round the north end, and this is the most rugged remote area of Bute.
A U-shaped valley running from Kames to Ettrick bays marks the line of a softer, shale-like rock called phyllites. In many places it is covered by layers of gravel and boulder clay left by the retreating glacier. Despite poor-draining properties it has been made into fertile farmland. It is unlikely that the sea has ever transgressed through this valley, even during the periods of high sea level that have left traces in raised beaches and clifflines well abobe present day shorelines.
A second valley cuts the island diagonally from Rothesay to Scalpsie along the line of the Highland Boundary Fault, and is occupied by a series of lochs. To the north and west of this line the rock is still of a schistose nature, and the landscape generally hilly though never high. A spine of hills running parallel to the fault reaches 160m at Barone Hill and extends down to Ardscalpsie Point, marking the southernmost outpost of the Scottish Highlands. This is an area of moor and heathland, with farming country towards the coasts. There is a fine extent of ancient woodland at Barmore, on the scarp above Loch Fad.
To the south and east the landscape is gentler. The bedrock here is Old Red Sandstone, as in parts of Ayrshire, and the soil more reddish-brown, but the glacial drift which overlies it in many places obscures the distinction, since it contains much material that originated north of the Fault. The ground rises to 120m on Scoulag moor, and is mostly farmland with many small woods. Tree cover is more extensive in the policies of Mount Stuart, which occupy much of the eastern shoreline.
A second valley between Kilchattan Bay and Stravannan Bay marks an area of softer sandstone. This isthmus is now a mixture of fertile farmland and conifer plantations. South of it, the landscape changes dramatically as lava flows which overlie the sandstone have resisted erosion and now stand proud, creating a rugged country of miniature mountains and cliffs entirely unlike the rest of Bute.
A legacy of glaciation which has left a profound mark on the island is the uplift of the land as the weight of ice melted off it, creating raised beaches all round the coast. These distinctive areas are often rocky and ill-draining and have proved resistant to agriculture. Although built upon down the east coast between Port Bannatyne and Ascog, the raised beaches elsewhere in the island are often rough and marshy, and provide a wonderful habitat for many kinds of wildlife. Behind them are cliffs, known as fossil cliffs since the sea is no longer eroding them, and these too are home to many interesting plants and fauna.
The influence of climate on the rocks and soils is the second great natural force which determines the nature of landscape and habitat. Generally, Bute’s climate can be summed up in the phrase ‘damp and mild’, but there are considerable local variations. Generally, the north resembles Argyll, while the south is like the Ayrshire coast. That winters are usually mild is shown by the palm trees (Cordyline australis) that flourish on Rothesay’s Esplanade and elsewhere. Winter mean temperatures at the south of Bute are slightly higher than on the Sussex coast, while wind exposure is much less severe than on islands such as Tiree and Colonsay where the winter temperature is c.0.5ºC higher. Summer temperatures are similar to other sheltered coastal areas of southern Scotland. Rainfall in the south of the island is c.40 inches (1000mm) annually, rising to nearly double that figure in the north. Sunshine averages c.1300 hours annually at Rothesay, and is a little higher further the south in the island.