Tag Archives: Formation Cliffs

Geological history and the Bedlam Walls area

For readers interested in the rock foundations of the Bedlam Walls Point the information from http://www.mrt.tas.gov.au/mrtdoc/dominfo/download/TR12_116_118/TR12_116_118.pdf may be of help.

“The area examined is rectangular and measures about 700 yards from SW to NE and about 600 yards from NW to SE. It is on an elevated promontory projecting into the Derwent estuary, bounded on the N by Shag Bay, on the W by the Derwent River and on the S by Geilston Bay.

The rocks present are fine light grey sandstone and siltstone of the Malbina Formation of Permian age. The sandstone forms hard, compact massive beds from one foot to six feet in thickness which alternate with siltstone beds generally less than two feet in thickness. The siltstone weathers more readily and so is readily distinguished in outcrop, but it is not markedly softer than the sandstone.

The rocks dip at low angles up to 6° to the W, that is, towards the Derwent. They possess vertical joints usually from nine inches to four feet apart although locally they may be as little as one inch or as far as six feet apart. Scattered isolated pebbles up to eight inches in diameter are present in the Formation Cliffs and on the W side of the area they plunge into the Derwent River. Along the foreshore in the NW corner are three shallow sea caves about 30 feet along any diameter. Their floors are now about twenty feet above sea level. There has been some collapse. No evidence of groundwater was seen either on the promontory or along the shoreline. The soil cover is a shallow silty grey loam overlaying thin stony clay. This forms a podsolic profile up to  eighteen inches and usually less than one foot in thickness.

Comments: In view of the rock types present, the low angle of dip and the well-drained nature at the rocks, the general stability of the area is not in doubt. Locally, the presence of the caves has been mentioned but they are superficial features caused by marine erosion, and do not extend underground for any distance. Except on the steepest slopes, the rocks are adequate to support normal industrial structures. Excavation will be assisted by the jointed and bedded nature of the rocks. Where there are large joint spacings, explosives will be required but in the closely jointed areas heavy earth moving equipment will be sufficient. Blocks up to six feet in diameter could be quarried from the massive sandstone. Groundwater supplies are not likely, and in any case would be limited in quantity to, at the most, 1,000 gallons per hour from a bore hole, and in quality by the proximity of sea water.”

The photo below was taken off the rocks at Bedlam Walls Point.