When you are at the posted coordinates you will be standing at the edge of one of the largest (if not the largest) potholes in New Hampshire. The huge pothole, known as the Pulpit, is similar to the Basin in Franconia Notch, but the Pulpit is much larger. It is partly buried by sediments, but the exposed part measures 42 feet deep and about 23 feet wide. Known in the past as Devil's Pulpit, the name originally referred to the rock that juts out over the pothole as a pulpit would jut out into a sanctuary. Over time the name as been used to refer to the pothole itself.
So how old is the pothole?
Pulpit Rock was formed by erosion of the bedrock at the end of a geologic time known as the "Pleistocene." The Pleistocene is the name given by geologists to the Ice Age, which ended in New Hampshire about 12,000 years ago. About 18,000 years ago, however, the glacial ice was more than 1 mile thick at this location.
Then about 14,000 years ago, the margin of the glacial ice melted northward through this area. This allowed a lake in the Coontoocook basin (Lake Contoocook) and one in the Piscataquog basin (Lake Piscataquog) to form right up against the edge of the glacier's margin.
So how was this pothole formed?
The pothole was formed by an incredible amount of water being forced through a very small area. The mystery of why so much meltwater flowed through Pulpit Rock is solved by examining other geologic sites to the west of here in the Contoocook and Piscataquog River basins. The geology of those areas allows for the reconstruction of the history of this area during the time when the last great glacier was melting back to the north. The following diagram shows the what the lakes probably looked like at the glacier's margin while the Pulpit Rock area was under ice.
Figure 1: Schematic diagram showing a possible ice-marginal position and the development of glacial Lake Contoocook and glacial Lake Piscataquog prior to draining through the Pulpit Rock area of Bedford, N.H.
When the glacier pulled back slightly due to melting, therby exposing the Pulpit Rock area, it opened a path along its margin for the water from the two lakes to flow down a new path which allowed for large volumes of meltwater to flow through the Pulpit Rock Conservation Area to cause the erosion that created Pulpit Rock. The Coontoocook River basin is the largest northwardly draining basin in New Hampshire so a great deal of water could have come through this area in a short time and it could have happened repeatedly as the glacial margin moved back and forth. A sudden glacial outflow is called a jökulhlaup from the Icelandic (jökull = glacier, hlaup = floodburst). The water volume can be very high; Peak flows as high as 15,000 cubic meters per second have been recorded in some jökulhlaups (six times the peak flow of Horseshoe Falls at Niagra). The diagram below shows what the area may have looked like after one or more jokulhlaups in the area (the missing water likely flowed through Pulpit Rock).
Figure 2: Schematic diagram showing a possible ice-marginal position just after the release of glacial- ice-dam-break floods down through the Pulpit Rock area of Bedford, N.H.
The pothole shape is caused by boulders and other sediments being swirled around and around by the force of the water as it cascaded down into the basin.
Potholes are not uncommon in rivers, how did such a large pothole get formed?
The existing watershed for the intermittent stream that sometimes flows into the pothole is around one square mile which means there isn't a lot of water than can flow into the pothole today. How did such a large pothole form without a large river here? Theories are as follows:
- This pothole (and a couple of other large ones downstream) probably formed at the base of a falls or cascade at or near the ice margin. The ice margin would have been necessary to provide a drop in elevation (hydraulic head) that isn't present in the land surface upstream of Pulpit Rock. Estimates are that the waterfall would have needed to be between 80-100 feet high to have sufficient force to create the pothole.
- They may have formed successively at different times and at different ice-marginal positions as the glacier's edge moved forwards and backwards.
- The ice margin must have remained in this area for a considerable period of time in order for these potholes to form.
- Periodic jokulhlaup (glacial outburst) floods contributed to the development of potholes (likely resulting from a series of minor advances and retreats of the ice margin in the two western basins.
Visualize!
Imagine being near this spot 14,000 years ago and looking at the waterfall coming off of a wall of ice and creating the pothole with the noise and the mist!
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- What are the estimates of how high the waterfall would need to be coming off the edge of the glacier to have created this size pothole?
- Standing at the GZ down in the pothole, estimate the width of the worn curved areas referred to in question 1 (including the current waterfall's path if there is water), or if you're not good at estimating, compare the relative size of the worn curved areas that you can see in the rock wall above the pothole.
- Standing at the GZ and looking at the waterfall; just above the pothole are half-circles worn into the bedrock where rock ground a circle into the bedrock. The bottom half of the circles are missing. How many half-circle shapes do you see?
If you have time, go downstream 50 feet or so where you'll see another very large pothole.
Pictures are welcome...as long as they don't give away the answers. :)
For additional information see the information sheet posted at the kiosk at parking. Much more detailed information may be found at:
Late Wisconsinan Deglaciation Styles of Parts of the Contoocook, Souhegan, and Piscataquog Drainage Basins, New Hampshire By Carol T. Hildreth and Richard Bridge Moore