Sunday, February 21, 2010

Is all this rain really that unusual?

It's raining hard once again today. So I thought I'd return to my theme of last Thursday and ask the question of how unusual this rain really is. So I went into my photo archive to help me. Below are three photos of fossil breccias. In the first photo it's the material that shows up as orange angainst the white limestone. This is actually fossilized mud, the read earth so typical of the Mediterranean. This site, Rosia Bay, was actually described by the Reverend John White in the 1770s and he wrote to his famous brother - Gilbert White of Selborne - about it. You can see the scale of the site in the second photo (two geologists in the middle for scale!). The third picture shows fossils (Red Deer and smaller animals) embedded. So what is this? - the site dates back to at least 100 thousand years ago and it was once a fissure that trapped the animals that grazed on the Rock above. Heavy rainfall then forced the mud into the cracks and fissures in the limestone and it solidified. By the way this is the type locality for the extinct rabbit Prolagus calpensis. We are still studying this site which continues to generate fossils. But for now, imagine the volume of water that must have fallen in the Middle-Late Pleistocene!






...and, of course, the wonderful cave formations inside the Rock were also shaped by lots of water filtering through the limestone




In Thursday's blog I mentioned the catchments that were made to trap rain water. Here's a wonderful early 20th Century view that clearly shows the recently devegetated area that became the catchment. If you compare it to my photo of Thursday you can see how far the vegetation has recovered its old ground.


I also mentioned later catctchments made of corrugated iron sheets over an ancient sand dune. The next two photos show how it was all done.

14th August, 1909 - laying down the sheets

July 1925 - you can see the timber frames dug into the sand on which the sheets were placed

The water collected was pumped into huge reservoirs excavated inside the Rock. This reservoir dates to 1898

They are still there, with some of the original kit it seems!


Nowadays they store desalinated water. The tunnel crosses the Rock from east to west!


Now, is all this rainfall unusual? I have trawled the old Gibraltar directories and have picked two incidents to post here:

17th November, 1834
Torrential rain (over 5 inches) damaged the streets and ramps. A number of houses were flooded, others washed away in whole or in part, and ten deaths ensued. Six bodies were taken out of some of the lower rooms in Castle Road, the unfortunate individuals having been killed in consequence of the immense fall of water from the Rock, which rushed into the premises, and overflowed them in such manner that, at one time, there were five feet of water in the rooms. Three more individuals were killed by the falling in of part of a house, near Castle Gully, caused by the same fall of water, and remained buried in the ruins; a child was, by the overflowing torrent, forced out of the hands of a young man who was endeavouring to rescue her from the perilous situation she was in, part of her little frock being left in one of them, and the body was carried from District No26 to the Public Baths (a distance of at least half a mile).

10th December, 1885
One of the most severe storms on record of thunder, lightning and rain burst over the Rock this afternoon. Great damage was done in every direction. At the Station Hospital, South, a landslip occurred and a wall and some small buildings were destroyed. At Catalan Bay the Barracks were flooded. The lightning struck the new wing of the Convent at the moment when His Excellency the Governor was in the Billiard Room. The shock was so severe that His Excellency was almost knocked down by it. A water pipe burst at Cornwall’s Parade flooding the neighbourhood. The inhabitants of a house in Willis’ Road, of which the back wall gave way, were got out by placing ladders against the windows. Also another house in Prince Edward’s Road was rendered uninhabitable by the inside walls giving way. The registered rainfall from 9a.m. yesterday to the same hour to-day was 6.90 inches.

So maybe it's not unusual...

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