Oddly enough I'm taking a Historical Geology class this semester. So far we've spent the first few weeks of the semester going over Physical Geology, such as the different types of rocks, so that we can have a better understanding of the fossil records. It's been an interesting class so far.
I picked up a piece of rock from the dredge pile last summer at the Blue Hole. Here's a picture from it:
It's hard to tell from the picture, but it is a sedimentary rock that is crystalline in nature. I has some medium foliation, and is gritty to the touch. Based on the other descriptions of the rocks within the crystalline group, I would say that it is either limestone or dolostone. My guess would be that it is limestone. I haven't actually taken the rock up to school to see if I can put some diluted hydrochloric acid on it to see if it effervesces though. My guess is that it would effervesce, which would mean that the glue holding the rock together is calcite, which would confirm that it's limestone.
The water that flows through the Blue Hole is also very hard, as noted by the white chalky film that it leaves on your gear after a dive. Hard water is made up of calcium carbonate (read calcite?), which is part of the building blocks of limestone.
Strangely, the rocks over at Perch Lake are different.
This rock is also crystalline in nature, but is very porous. In fact, if you rub your finger lightly against the rock or thump it, it has a very hollow sound to it. (Almost like thumping a glass bottle) I think that this rock is also limestone. The porous nature of the rock may be because Perch Lake doesn't have any noticeable flow to the spring. The lake is lined with this rock along the walls where it drops off from around the 20' mark to 40'. I got this rock just south of the car at a rock uprising. I believe that the water seeps through this rock to form the lake as we know it. I'm not entirely sure, but the silt at the bottom of Perch Lake may be volcanic ash.
I do agree that the lack of igneous rock could mean that the holes were not formed by volcanic lava tubes. I'd be interested in taking more rock samples.
Given the presence of limestone, gypsum, and even the calcium carbonate from the hard water could mean that the area surrounding the Blue Hole could have possibly been a shallow sea at one point in time. If you look closely at the rock from Perch Lake you will see a shell that has been fossilized in the rock. Limestone is made up of the remains of shells from sea animals and gypsum are deposits of hydrated calcium sulfate from dried up sea beds.
Makes you think.
