This short article discusses the effects of temperature on speleothems found near cave entrances.

Square Cross Section Straw Stalactites

Andy Spate (pers comm) once mentioned that at Yarrangobilly Caves, he has seen straw stalactites with a pronounced square cross - section. These apparently occur near the entrances of some of the colder caves. The climate at Yarrangobilly Caves is cool temperate, with snow in the winter and warm summers.
Without having seen these stalactites, I would venture to suggest that the square shape might be caused by water on the stalactite freezing at times, expanding and fracturing the tip of the straw where the water drip normally sits.
I have seen broken straws from Jenolan Caves. Although the straws have an overall circular cross-section, they also have two acute points corresponding to the calcite cleavage, forming a diamond or square shape depending on the viewing angle and the cleavage plane.

Fractured, Truncated Stalactites and Stalagmites

At Wombeyan Caves, the self - guided tour of Fig Tree Cave takes the visitor through the large Fig Tree Cave, then into the Creek Cave via a section known as the Collonades. Just past a large fallen stalactite, the visitor may observe near the ceiling several stalactites with a pronounced flat underside. Each of these stalactites has a deposit of cave coral near the broken end.
If the visitor continues along the narrow path, a number of other broken speleothems may be observed. There are some broken wind - deflected stalagmites and stalactites. One broken stalagmite is at knee level; another is down at creek level.
One could imagine that one of the stalagmites was broken by track development, however in this case I think it may have been broken by the effects of flood. The broken stalactites are all way out of reach of people, and positioned in the middle of the passage near the roof. The present creek level is very low, about 15 m below the stalactites and about 10 m below the stalagmites and pathway. The efflux of the creek is via another cave system with various restrictions along the passage. There is an overflow efflux which becomes active during floods. During historical flood events, the effluxes temporarily block with debris, causing the creek to bank up in the cave. There is a log in the cave perched high and dry at about the same level as the broken stalagmite, so it would be reasonable to assume that the stalagmites were broken by flood debris such as whole trees smashing into them.
However it seems unlikely that a flood event could fill the whole cave; obviously one would need to seek more evidence that the cave filled to the ceiling but there is another possibility regarding the broken stalactites, and that is temperature.
Consider the stalactite in unbroken form, with a wet tip. Now consider that it is late winter, and icy. It still snows at Wombeyan occasionally. Suppose the wind blows through the Collonades, from the outside as it still does at times. This may freeze the water on the tips of the stalactites, causing them to develop fractures wherever the water can seep in, such as around the coralloids.
The ice may build up, especially if the stalactite is active. The average temperature at Wombeyan is around 16 ˆC so the base of the stalactite would be about this temperature with a graduation to the tip where our hypothetical ice is developing.
If this scenario is repeated a few times, the stalactites may simply fail along a fracture, leaving the present stub.