MESAC NEWS

Erosion and what is can tell us about geological history

SEEP

Sedimentary rocks that were deposited in the sea are the only rock types exposed at Rickett’s Point. These rocks were formed in three ways: as products of weathering and erosion of earlier rocks, from the remains of organisms that had mineralised shells, and from chemical precipitates.

The most abundant rock type at Rickett’s Point is the Beaumaris Sandstone, which is exposed on the shore platform as dark and mid brown alternating beds. Some five and a half million years ago, following compaction and cementation of the sediments that comprise these rocks, they were uplifted and eroded. But erosion was not uniform. The more compacted and cemented the rock, the slower the erosion. At mid tide, in front of the Rickett’s Point Tea House, is an elevated structure that has eroded less than the surrounding rocks. In trying to determine why this happened, one needs to consider whether the sediments that comprise this structure were more compacted, or more cemented. It is the latter process that is the more likely.

But how?

After deposition, groundwater moved through the partially compacted Beaumaris Sandstone sediments. At one site, the flow of groundwater, and the minerals dissolved in it, was greater than in the surrounding sediments. These mineralised fluids would have exited the sediments as a cold seep, and in doing so, they filled many of the small gaps between sediment grains. They thus made the sediments in the seep area more cemented.

Following uplift, the erosion of rocks on the platform began, but at the site of the cold seep, erosion was less effective, resulting in the structure preserved there today.

Written by Dr. John Buckeridge 2026