Why Is There a Round Pond in the Middle of Flat Farmland? A Block of Ice Melted
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A steep-sided hollow with no inflow or outflow in otherwise level ground marks the spot where a buried lump of glacier ice sat while sediment piled up around it.
How one forms
A retreating glacier leaves detached blocks of ice behind, some of them tens of metres across. Meltwater streams pouring off the ice front carry sand and gravel and bury those blocks partly or completely. The buried ice is insulated and melts far more slowly than exposed ice would, sometimes taking centuries. When it finally goes, the sediment above and around it has nothing to rest on and collapses inward, leaving a steep-sided hollow in ground that is otherwise flat outwash plain.
How to recognise one
The features are distinctive once known:
- •A closed depression with no stream entering or leaving
- •Steep sides relative to the surrounding flat ground
- •Roughly circular or irregular rather than elongated
- •Frequently holding a pond fed only by groundwater and rain
- •Occurring in groups across a former glacier margin
- •Set into sand and gravel rather than into bedrock
Why they matter ecologically
These hollows are ecologically important out of proportion to their size, and the reason is isolation. A pond with no inflow or outflow receives no fish unless somebody puts them there, which makes it a refuge for amphibians and invertebrates that fish eat, so populations survive in them that cannot survive in connected water. Many dry out in summer, which excludes fish permanently and suits species adapted to temporary water. The prairie pothole region of North America consists of millions of them and produces a very large share of the continent's ducks.
How they are told apart from craters
A circular hollow has several possible causes and distinguishing them matters before anybody draws conclusions. An impact crater has a raised rim of material thrown outwards, shattered rock beneath and frequently a central peak, none of which appears here. A sinkhole in limestone sits in soluble bedrock and usually drains freely downwards, where these hold water. A bomb crater has the same raised rim and appears in patterns along a line. A pond dug by people has straight edges or a regular outline and usually a dam or a cut channel. The setting in sand and gravel at a former ice margin is the deciding evidence.
What the sediment in them records
Because they have no inflow, the sediment accumulating on the floor comes almost entirely from the air and from the immediate surroundings, which makes it an unusually clean record. Pollen settling year after year records what was growing nearby, so a core through the floor reconstructs the vegetation history of the area since the ice left. The bottom of that sequence dates the melting of the buried block, which in turn dates the retreat of the glacier. Volcanic ash layers and charcoal from fires appear in the same cores and tie the local record to events elsewhere.
The takeaway
A block of ice left by a retreating glacier is buried by meltwater sediment, melts slowly under its insulating cover, and leaves a steep-sided closed hollow when the ground above collapses. With no inflow or outflow, the resulting ponds hold no fish and shelter amphibians accordingly. Pollen settling on the floor records the vegetation history since the ice left.