Piprake
November 19, 2006
Provided by: Ray Murphy
This photograph shows a pipkrake very early on a winter morning. It was taken close to an iron-rich groundwater seepage near Ste-Lucie, Quebec, Canada. The air temperature was below freezing, but the temperature below the surface of the ground was still above the freezing point. The length of the ice needles is about one and one-half inches (about 2 cm). I first heard the word "pipkrake" defined as "an ice stem, carrying soil." The photograph clearly shows that this pipkrake formation is indeed a soil transport mechanism -- particles of the reddish soil are lifted as the pipkrake grows.
The slope of the ground at the seepage is about 60 degrees from horizontal and defines a local hydraulic equipotential surface. Groundwater flow, as evidenced by the continuous action freezing of the extruded groundwater into the pipkrake, is clearly perpendicular to the local surface. Note that the slight upward curve of the ice is most likely due to a slightly greater pressure and flow rate at the bottom edge of each forming needle. The picture was snapped just as the Sun started illuminating the scene (this formation melted a short while later).
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