The rock of Gibraltar, famous for its strategic position at the mouth of the Mediterranean has been fought over for centuries, its precipitous surface rising from a glittering sea, a veritable fortress of parapets and defensive walls. It is an 1800 foot high slab of limestone; riddled with caves and tunnels. The rock's defenders living in them for sometimes years at a stretch.
This particular photo is of a spectacular display of flowstone that I had noticed deep down in the bowels of the St. Michaels Cave. It is the largest and most well known system on the rock. Prehistoric humans are said to have first ventured there. Today tourists visit a huge amphitheater in the Upper Tunnels; said to have been a Roman temple at one time and an operating theatre during World War Two. Deep down, past cavernous pits, along hanging ropes I was fortunate to have priviledged access to the lower tunnels (more on them in a later post).
Flowstone deposits such as these are generally comprised of some carbonate mineral, calcite being the most common. Though it is debatable some scientists speculate that calcite is most often deposited in cooler climates and aragonite in the warmer areas.
Speleothems precipitate from a supersaturated solution in many amazing forms. Most well known are the delicate soda straws; hollow tubes that hang from the roof in widths up to half an inch across. It is a caver's nightmare to break one as they take thousands of years to form and one careless second to snap. These fragile tubes grow in length by the depositions of water droplets that run down its hollow central channel. If the straws become blocked the formation continues to grow in girth by deposition over its outer skin; a carrot shaped spike evolves from the process. On the floor beneath stalagmites stretch upward, the falling droplets feeding the growth of their squat indelicate humps.
Speleothems form in a myriad of other odd permutations. Shields fan outward along the edges of a bedrock fracture. Where groundwater is extruded from the crevice under hydrostatic pressure, the deposits form as plate-like sheets. Anthodites (cave flowers) blossom in radiating spines of aragonite, draperies hang down in wriggling folds of stone and cave rafts grow on mirror-still pools. A whole strange world of sculpted limestone wonder develops in the inky blackness.
In Ontario, the province in which I normally cave, the tunnels have generally been formed over the last 14 000 years – new in geological terms. Cave passages are close to the surface in relatively horizontal strata. James Sled had told me of a relatively deep cave at the bottom of a pit though I am yet to find it. Most tunnels are still in the "excavating" process and decent speleothems are a rare commodity.
Needless to say, it came as a great surprise to the Ontario caving community when a local cave was discovered dripping with speleothems of a variety and quantity unknown elsewhere (in the area). From a low crawling entrance of cobbled rock cavers entered a gallery of incredible beauty. Stalactites hung down to 8 metres in length and beneath the decorations there is an ancient beaver's nest entombed in calcite. It is thought that these animals must have lived here when the water level was at the unusually high level of post-glacial Lake Nipissing.
Most unusual of all was the discovery of live cedar's roots that grew through the top of the cavern. Its roots are encased in calcite and form part of the central mass of a great column of deposition that cascades as a frozen stone waterfall from the roof. Some study has been done in reguard to the possibility of some bacterial action on the roots that has increased the rate of speleothem growth. It is from this wonderful feature that the passages have taken their name – "Root Cave"
CHECK OUT MY BOOK AS DISPLAYED IN THE SIDEBAR TO THE RIGHT (ROCKWATCHING; ADVENTURES ABOVE AND BELOW ONTARIO) I DISCUSS MANY AMAZING LOCAL CAVE DISCOVERIES AND HIKES IN WHICH YOU CAN OBSERVE THE FASCINATING LOCAL GEOLOGY.