South Fork Toutle River Bridge Area

Schematic stratigraphic section in Harry Gardner Park. L, lithic-rich layer; P, pumice-rich layer.
Modified from: Doukas, 1990, Road Guide to Volcanic Deposits of Mount St. Helens and Vicinity, Washington: U. S. Geological Survey Bulletin 1859

Here, the outcrops, more than 20 feet high, consist of lahars of the Pine Creek eruptive period (3000-2500 years ago). One lahar here contains unusually well rounded clasts and monolithic megaclasts of silt-size material. This lahar has been interpreted as a breakout lahar, formed by sudden breaching of an ancient Spirit Lake (Kevin Scott, 1985). A lahar would have been able to carry the fragile megaclasts of silt, whereas a stream would have destroyed the clasts. To the right of the high cliff are deposits from the May 18, 1980, mudflows that overlie asphalt paving of the Harry Gardner Park parking lot. The lower, lithic-rich layer represents the first and largest lahar that came down the South Fork Toutle River, arriving here at 10:13 a.m. P.D.T. on May 18. The upper, pumice-rich layer represents deposits from a second lahar in the South Fork Toutle River. This pumice-rich lahar reached here in the evening of May 18 and was dammed by thick deposits of the lahar from the North Fork; the pumice came from the plinian eruption that began around noon.

Excerpt from: Doukas, 1990, Road Guide to Volcanic Deposits of Mount St. Helens and Vicinity, Washington: USGS Bulletin 1859