Know your Islands
Wave cut terraces are a narrow flat area in the rock caused by the action of waves. It forms after destructive waves hit against the cliff face, causing undercutting between the high and low water marks. This notch may then enlarge into a cave. A wave cut platform represents an extremely hostile environment, where only the toughest of organisms can live.
By using scientific dating methods, or examination of marine fossils found on the platform, it is possible to work out when the platform was formed, thus giving geographers and geologists information about sea levels at known times in the past.
The following is taken from ‘Islands from the Sea: Geologic Stories of Cayman’ by Murray A. Roed, available at the National Trust – 949-0121,
Six levels of wave cut terraces are recognized in the Cayman Islands, some from aerial photos, some from scuba divers’ observations plus bathymetry, and some found by previous workers.
The terrace story starts at the peak of the Late Wisconsin glacier build-up, ignoring minor fluctuations. This happened approximately 18,000 years ago. At that time sea level is believed to have been up to 130 meters below present sea level.
On Grand Cayman, this level is represented by a prominent wave cut notch at a depth of about 120 meters along a steep bedrock wall 200 meters offshore near Northwest Point in West Bay. Other evidence of this low sea level may be represented by tunnels in the bedrock and tall bedrock pinnacles, some 30 to 40 meters high, described by divers as ‘haystacks’.
These conspicuous sub-sea relics are composed of carbonate rocks, and have been recorded from 230 to 310 meters depth off the western margin of Grand Cayman. The pinnacles are either eroded bedrock or, more likely, they are rockslide debris.
The next recognizable terrace level is a narrow terrace at a depth of approximately 70 meters below present sea level. This is at least 50 meters above the deep terrace level and likely represents the first major melting phase of the continental glaciers that had occurred by about 15,000 years ago.
A lot of water was added relatively quickly to the oceans in this interval that may have spanned about 3000 years. A pause or slowdown in melting provided enough time to erode this narrow terrace. Air photos indicate suggestions of this terrace in only a few places, off Grand Cayman and off Cayman Brac near Hawkesbill Bay. Other scientists have reported a break in slope here off Grand Cayman in a number of places.
The next terrace is a major break in slope recognizable off all the islands at a depth of about 40 meters. The outer limit of this terrace edge is up to two kilometers offshore, but commonly it is less than on kilometer, and in places as close as 200 meters from shore. This surface slopes shoreward up to about 12 per cent and ranges in width from 100 meters to over 1000 meters. The outer edge of this terrace is the beginning of the main ‘wall’ of the offshore, literally the edge of the shelf, or the drop into very deep water. This well formed terrace was eroded during a prolonged interval after which much of the Continental ice had melted, perhaps by about 12,000 years ago.
Please see Chapter 6 of this book for more information on three other wave cut terraces
Last week’s answer: Never evict bats during June, July, August, September, October or November when baby bats would be separated from their mothers and left to die. This is unwise, as bats bear only one baby a year and an entire generation would be lost.
Trivia question: What is the highest point of Grand Cayman?
Look for the answer in next week’s feature!