Socio-economic development in lowland post-glacial environments

Overview

Drainage problem in a drumlin landscape and drawbacks of outwash land for farming.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why is a lowland post-glacial landscape often good farmland?

   Model answer

   Because glaciation left behind deep, fresh, mineral-rich soil parent material where previously there might have been bare or thin-soiled rock. Till is a ground-up mixture of many rock types, so it weathers to a soil with a wide range of nutrients and, where limestone was among the rocks crossed by the ice, a useful base status that resists acidification. It is deep, stone-free enough to plough in most places, and holds moisture well in a dry summer because of its clay content. In a drumlin landscape the well-drained convex tops and upper slopes are the best of it, and such areas typically carry improved grassland for dairying and beef, with mixed arable on the gentler ground - the pattern found across the drumlin belt of County Down.

2. 2

   What is the drainage problem in a drumlin landscape, and why does it arise?

   Model answer

   The hollows between the drumlins are waterlogged for much of the year, occupied by rushes, marsh, small lakes and peat, and they can be too wet to graze or cultivate. The problem arises because the till from which the landscape is built is clay-rich and was compacted under the weight of the ice, so it is close to impermeable and water cannot soak away downwards. Glaciation also disrupted the pre-existing drainage network, so surface water has no organised route out, and the closed hollows between the mounds simply collect it. Rainfall in a maritime climate exceeds evaporation for much of the year, which keeps the water table at or near the surface in the low ground.

3. 3

   How do farmers deal with waterlogging in a drumlin landscape, and at what cost?

   Model answer

   By artificial drainage: cutting open ditches or sheughs along the field margins, laying networks of perforated underdrains, mole ploughing to open temporary fissures in the clay, and in some districts arterial schemes to deepen and straighten the receiving watercourses. The cost is substantial. Installation is expensive per hectare and, because the clay reseals and drains silt up, the work has to be repeated every generation, so it is a recurring rather than a one-off investment. Only farms of a certain size and profitability can justify it. There are also environmental costs: draining the hollows removes wetland habitat, fen and small lakes of high conservation value, and speeds runoff into rivers, which can increase downstream flood peaks.

4. 4

   What problems do the slopes of drumlins cause for farming?

   Model answer

   The flanks can be steep enough to make machinery awkward and, on the steepest, unsafe, so cultivation is difficult and some slopes are left in permanent grass. Where they are ploughed, the combination of slope, heavy rainfall and a silty topsoil produces soil erosion and rill development, and thin soils on the crests dry out quickly in summer. The shape of the ground also imposes a field pattern of small, irregular, awkwardly shaped fields following the contours of individual mounds, which is inefficient for large modern machinery and helps explain why farms in drumlin districts tend to be small, fragmented and scattered rather than large and consolidated.

5. 5

   What effect does a drumlin swarm have on routeways?

   Model answer

   A very obstructive one. Because the mounds are numerous, closely spaced and separated by wet hollows, a road or railway cannot take a direct line. It must either climb over the mounds, giving a switchback route with repeated steep gradients and blind summits, or wind between them on a longer, sinuous course, or cross the boggy hollows on expensive embankments. All three raise construction and maintenance costs, lengthen journeys and reduce the standard of alignment that can be achieved, which is a real difficulty for modern dual carriageways and railways that require gentle gradients and curves. Minor roads in drumlin country characteristically follow the low ground and twist repeatedly.

6. 6

   How do engineers cope with drumlin country, and what are the trade-offs?

   Model answer

   Either by working with the terrain or against it. Working with it means routing the road along the hollows and around the mounds, which is cheap to build but gives a longer, slower, more winding route on ground that is wet and needs drainage. Working against it means cutting through the drumlins and embanking across the hollows to achieve a straight, level alignment, which gives a fast modern road but is expensive in excavation, in disposal or reuse of the spoil, and in land take, and it produces cuttings in unstable till that need shallow slope angles and drainage to prevent slumping. In practice a compromise is used, and the excavated till is often reused as embankment fill to reduce haulage costs.

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