河床上的碎石大部分是由风化作用产生的碎屑物质。

Most of the gravel in the riverbed is ___detrital material formed by weathering.

森林地面覆盖着大量由落叶和枯枝组成的有机碎屑。

The forest floor is covered with abundant ___detrital material composed of fallen leaves and dead branches.