If the ¹⁰Be concentration in littoral sand is 1x10⁵ atoms/gram quartz, that in river sand is 1x10⁴ atoms/gram quartz and the mean ¹⁰Be in terrace deposits atop the backwearing cliffs is 2x10⁵ atoms/gram quartz, what proportion of the littoral sand is derived from the rivers and the cliffs, respectively. Assume that the only contribution to the littoral sand from cliff backwearing is due to the terrace deposits themselves – in other words, the bedrock of the cliffs produces no sand.