When the official temperature dropped below 30 yesterday, it marked the first time since last Feb. 9-11 that it had fallen into the 20s in Philadelphia on three consecutive days.
That cool spell is over, but it has left a subtle legacy. Ice again is appearing on the rocks along the Schuylkill Expressway between Gladwyne and Conshohocken.
This is a common wintertime phenomenon, and while it may seem a prosaic development to the casual observer, we note that this is all part of immense and almost unimaginable processes.
They are part of what Temple University geologist Laura Toran has called "groundwater outcrops."
Here is how geologists explain it:
Those rocks, knowns as Baltimore gneiss, are the leftovers of a billion-year-old collision dating to the time when the Earth had only one land mass.
Over time, fractures have developed in the rocks, allowing groundwater to seep through. The blasting for the expressway probably widened those fractures.
When it's cold enough, the oozing water freezes, and as a cold spell continues, the ice formations grow and grow.
It is not clear when they will become more impressive. It may make a run at 60 tomorrow, and the long-term outlooks aren't bullish on prolonged cold.
In its morning briefing, the Commodity Weather Group, in Washington, said that the upper atmosphere may be showing signs of a pattern change.
But it might not happen until January.