Ah! But perhaps it wasn't meant to contain water, just to provide a conduit it to fill as it rose above the water table. In his book "The Silbury Treasure" Michael Dames writes:

<i>" In autumn and winter, the upper Kennet disappears underground and the two springs are dry - summertime surface evaporation on the downs having caused the underground saturation to sink below the Plenus marl layer. Not until this level has risen again, during the next low-evaporation winter, does the river re-appear. Its revival in February - March coincides with the general revival of life in spring. <b> In Neolithic times it seems likely that the period of total Kennet failure was of shorter duration than in an average modern winter, because of the advance of "scarp foot" springs in the clay vales flanking the downs."</b> </i>. Perhaps there was more water about then, and silt may not be an issue if it just came and went through the bottom of the ditch?

Pilgrim