The immense forces exerted by glaciers cause melted water to create tunnels.
A subglacial tunnel is a form of valley that occurs beneath the flow of a glacier. They generally occur near the fringes of large continental ice sheets, where the temperature has become sufficient to create large amounts of melted water. These valleys are often vast, extending up to 60 miles in length and 2.5 miles in width. The sides of subglacial tunnels are often asymmetric, unusual for something formed by erosion.
Formation of Subglacial Tunnels
Subglacial tunnel valleys form when ice at the edge of a glacial moraine begins to melt and the melted water has nowhere to go. This water, along with debris, which accelerates erosion, drains away into the land below the glacier, creating a tunnel extending back along the course of the glacier. This tunnel is where the glacier’s waste water and sediment are deposited. Studies of subglacial tunnels formed thousands of years ago have shown they were created by water and debris flowing uphill, which could only have been formed in the presence of the immense force of a glacier (see Reference 4).
These tunnel valleys have been shown by studies to form anastomosing networks as opposed to dendritic ones, which means subglacial channels are created in multiple locations and intersect each other, rather than growing out from one location like a tree. This is expected behavior considering that the valleys begin at the bottom of the glacier, which is the thick end of the wedge. Melted water produced at that point will be spread over a large area and will create numerous, independent channels.
Buried Subglacial Tunnels
The immense pressure subjected on water flow beneath a glacier means that water and debris are immensely pressurized. When this enormously pressurized water comes into contact with soft bedrock, it erodes down into the rock sharply, which causes a sharp spike in the depth of the valley, out of sync with the broad flat-bottomed valleys usually created by this process. Because debris and till are being deposited over a smaller area in these narrow valleys, they often become semi- or totally buried by debris.
Open Subglacial Tunnels
If the water and debris flow across a landmass of uniform resistance to erosion, a more standard broad U-shaped valley will be created. As till and sediment are deposited over a much wider area in these valleys, they are much less likely to become buried by debris. Instead, the debris left forms sedimentary layers that becomes part of the landscape. The forces pushing the melted water and debris along are often so great that these sedimentary layers are disrupted, as was evidenced in studies conducted on tunnel valleys in the Stony Brook and Port Jefferson areas of New York State.