South Sister St. Marys, Tasmania
- O.G. Ingles
Four soil samples were taken and subsequently inspected microscopically. Sample A was taken from an erosion/shallow slip feature on the Dublin Town Road; sample B from downslope of a standing pool on the Derricks Marsh road (about 15m from road); sample C from the topside of the pool. All three were taken from the top 20cms of the gradational soil profile, which is generally less than 100cms thick. The fourth sample was soil in suspension (as turbid water), taken directly from standing water in the pool, which had been recently disturbed by passage of an official vehicle. The local rock is an unusually fine-grained basaltic dolerite.
Samples A, B & C were identical in showing dispersion and no slaking when exposed to water without use of dispersants. The turbid water of sample D settled to a fully clear condition in less than 2 hours, indicating a mean particle size of around 5 microns (i.e. a fine silt).
Under the microscope, samples A, B & C all appeared to consist of fine particle aggregates with a median aggregated size around 20 to 50 microns (i.e. coarse silt); the fine particles being opaque silica grains with ferruginous coatings - that being the probable reason for their bonding into aggregates. No clay was detected in any sample (presumably washed out whenever formed, as the aggregates would form a soil with permeability of approx. 10-3 cm/sec, which is moderately high, and would account for high water uptake into the underlying rock strata, for which this soil would be a natural filter).
It follows that (A) if these natural aggregates are broken down into their constituent particles by heavy traffic, soil permeability would be irreversibly reduced to ~ 10-5 cm/sec, and the water intake into the underlying rock strata thereby seriously diminished; and (B) that in their natural condition, these aggregates fall close to the maximum erodibilty size - 200 microns - and would be likely subject to serious erosion under intense rainfall unless protected by dense understory growth binding this soil with its roots.
O. Ingles
9 April 2005
5453 (1, 14, 30, 66)