Where you might expect a burial mound to rise upward, this one effectively sinks. A pond barrow is a prehistoric funerary monument defined by its inversion: rather than a heaped central cairn, the interior is deliberately hollowed, creating a bowl-shaped depression enclosed by a surrounding earthen bank. The effect is the opposite of the conventional mound, and the reasoning behind it remains only partially understood, though the form is associated with Bronze Age ritual and mortuary practice elsewhere in these islands.

The Tonavane example is a substantial one, measuring roughly 42 metres north to south and 37.5 metres east to west overall, with an internal diameter of around 22 metres by 17.5 metres. The enclosing bank stands about a metre high externally and runs to approximately ten metres in width, and the floor of the interior drops some 1.5 metres below the crest of that bank, giving a clear sense of how much material was scooped from the centre and thrown outward to form the enclosure. No ditch, internal or external, was identified during survey. A nine-metre gap breaks the bank on its eastern side, most likely the result of a farmer pushing that section inward during field clearance at some point, an entirely understandable act given that the surrounding land has been heavily cultivated over time. Despite the disturbance, the site resolves into an almost perfect circle on aerial photography, which is often how such denuded earthworks reveal their true geometry when they have been reduced to near-invisibility at ground level.

Walking the field today, the hollow reads as a shallow, grass-covered bowl, easy to overlook or mistake for a natural dip in the land. The bank, worn as it is, still traces most of its circuit, and standing inside the depression gives a reasonable impression of the original proportions, even if the monument has lost much of its height to centuries of cultivation.