With a surface area of 70 kilometres, Loch Lomond is the largest and most accessible of all Scottish lochs and has a great diversity of environments and fish species. It lies only 7.9 metres above sea level and is used for water supply and for recreation. The present size of Loch Lomond is testimony to the erosive power of the glacial ice sheets that covered Scotland. It formed one of the main corridors for movement of ice away from the centres farther north down to the Clyde, and the ice excavated a trench probably involving removal of up to 600 metres of bedrock. In its northern part where the ice eroded more powerfully, the loch is narrow and deep (153m). Smoothed bedrock surfaces, ‘hanging’ valleys and steep landslipped valley sides can be seen around Inveruglas. In its wide, southern, lowland part where deposition of glacially-derived till, sands and gravels and clays has been dominant, the loch rarely exceeds 30 metres in depth. The geology is strongly reflected in the scenery found along its length. The softer Devonian conglomerates and sandstones at the south end have been weathered, eroded and scoured by the ice more easily than the harder metamorphic Dalradian rocks farther north.
The Highland margin is marked by steeply dipping Devonian conglomerates and the Highland Boundary Fault zone on Conic Hill above Balmaha and on the elongate islands of Inchmurrin, Creinch and Inchcailloch. Dalradian rocks are dominant north of the Highland Boundary fault and consist of metamorphosed sandstones, grits and mudstones with minor volcanic ash.
Around 13,000 years ago, as the ice melted, Loch Lomond became an arm of the sea. Borehole cores taken from the lower part of the Endrick valley show that laminated clays and silts were first deposited followed by massive, silty clays with marine shells and dropstones derived from ice rafts. They indicate that although the climate had become warmer, the winter conditions were still colder than those today. As overall uplift of the land continued, following ice removal, sea level fell rapidly and by 11,000 years ago lay at around the present level or below. With the onset of glaciation again between 11,000 and 10,000 years ago, a 20 to 50 metre thick sequence of marine muds was deposited ahead of the advancing ice.
This ice ‘bulldozed’ the existing deposits in the loch to form a prominent moraine that can be traced easily around the Loch Lomond area. It forms a prominent ridge up to 25 metres high stretching from Shantron Muir and Glen Fruin to Alexandria and then eastwards to near Killearn and north to Drymen. Glacial meltwater flowed down the River Leven depositing sand and gravel which effectively raised the outlet of the loch and are partly responsible for it being freshwater today. Beneath the ice a moundy lodgement till consisting of clayey and sandy material with many stones and boulders was laid down. The ice also dammed up the valleys of the Fruin, Endrick and Blane waters causing the formation of lakes. The sandy deposits of the terminal moraine southeast of Drymen pass southeastwards into the carved clays and silts of the former glacial lake of Strath Blane.
Around 10,000 years ago a rapid change to warmer but wet climatic conditions occurred and the ice disappeared from Scotland. In the succeeding period until 6,500 years ago temperatures rose to be warmer than at present and sea levels around Loch Lomond oscillated with marine conditions only prevailing at greater than 9,500 years ago and briefly around 7,900 years ago. Between 6,800 and 5,500 years ago a longer marine phase occurred and the remains of a series of old shorelines have been recognised at 13, 12 and 9 metres above sea level. Generally muddy sediments were deposited except at river mouths where sandy deltas formed. The Loch Lomond ‘herring’ or powan, is a char more typical of colder water today – a reminder of the loch’s recent glacial history.
The sediments on the bed of Loch Lomond together with other lochs in this area, and the local peat bogs, contain pollen grains and other plant remains that provide evidence of changing vegetation during the last 13,500 years. At the end of the late Devensian glaciation initial vegetation was of open habitat, with grasses, sedges and mosses taking hold on the bare soil. In some areas heath, dwarf shrub and some birch, willow and juniper grew but the general tree line did not reach the area. During the Loch Lomond Re-advance vegetation was open tundra and soils were destroyed as the plant cover broke up under the intense cold and dry conditions. Mugwort was an important plant along with some sedges and grasses. Ten thousand years ago at the end of the Ice Age heath and birch woodland spread rapidly, followed by pinewoods, with hazel and oak well established by 6,500 years ago.