The Great Trossachs Forest – A Glaciated Landscape

1 Geography: Glaciation

The landscape of The Great Trossachs Forest (TGTF) has been shaped by many different forces both natural and human.

The Highland Boundary Fault is nearby which runs from Arran to Stonehaven marking the boundary between the highlands and lowlands of Scotland. The Fault line is the result of the collision between two continents that occurred over 400 million years ago pushing up the mountains of Scotland. The fault line can be seen crossing Loch Lomond as a series of islands, and passes by David Marshall Lodge in Achray Forest, just south of TGTF.

Glaciation history

For the past 2.6 million years the earth has been going in and out of ice ages. Around 22,000 years ago the last ice age was at its height. At this time most of Scotland and much of England was covered in an ice sheet 1.5km thick (around the height of Ben Nevis, our tallest mountain). This glaciation finally came to an end around 11,500 years ago, after a period known as The Loch Lomond Stadial (because evidence for it was discovered around Loch Lomond!). This was a time when warming was suddenly halted and freezing conditions returned for 1500 years.

The ice sculpted and changed the area diverting whole river systems – Loch Arklet occupies a broad U-shaped valley, which once carried water from the west eastward into the Forth river system. During the last ice-age, ice flowing down from the mountains to the north of Loch Lomond, scoured out the shape of the Lomond valley and cut off this river system from the land to the west.

Glaciated Landscape features

The ice has had a major impact on the landscape of Scotland and many of the typical features of a glaciated landscape can be seen in TGTF. Strath Gartney where Loch Katrine is situated is a typical U- shaped valley eroded by glaciers. The effects of the ice can be seen along the sides of Strath Gartney as a series of truncated spurs and hanging valleys. The burns that run into Loch Katrine are examples of misfit streams - running in wide U-shaped valleys that they did not erode.

© Shirley Leek

The picture above shows Ben Venue seen from Ben A’an showing the rough sides of the mountain where it has been scoured by glaciers, a process called ‘abrasion’ - where rocks are picked up by the ice (‘plucking’) and dragged across the surface of the landscape.

Higher up in the mountains surrounding Loch Katrine there are examples of other glacial features:

1 Geography: Glaciation

Corries - armchair shaped hollows high on a mountainside where glaciers formed. This one is filled with trees.

Arêtes - where glaciers have eroded both sides of a ridge to produce a narrow knife-edge.

On the lower ground there are till deposits, where material taken from higher up has been deposited by the ice. These include moraines, which are areas of hummocky, uneven ground. Near Callander there is a well preserved ‘Terminal Moraine’ which marks the edge of a glacier that existed during the Loch Lomond Stadial.

At the top of Bochastle Hill there is a large boulder called Samson’s Putting Stone. According to local legend it is the result of a putting competition between a family of giants (Samson the giant won!). It is a glacial erratic – a boulder that was picked up by the ice and carried by the glacier before being deposited where it now sits.

1 Geography: Glaciation