Fig. 100.
- a. Vegetable soil.
- b. Alluvium.
- c. Mass of same, apparently detached.
The inferior surface of alluvial deposits is often very irregular, conforming to all the inequalities of the fundamental rocks ([fig. 100.]). Occasionally, a small mass, as at c, appears detached, and as if included in the subjacent formation. Such isolated portions are usually sections of winding subterranean hollows filled up with alluvium. They may have been the courses of springs or subterranean streamlets, which have flowed through and enlarged natural rents; or, when on a small scale and in soft strata, they may be spaces which the roots of large trees have once occupied, gravel and sand having been introduced after their decay.
Fig. 101.
Sand-pipes in the chalk at Eaton, near Norwich.