The Geologic History of the Diamond Lake Area

THE INTRACANYON BASALTS

Some time after the growth of Pig Iron Mountain and Watson Butte (which began about 12 million years ago), and the retreat of the last glaciers (which occurred at least 10,000 years ago), lava flows unlike any others in the area poured out of the earth's crust several miles to the north of Diamond Lake. These are the intracanyon basalts of the Clearwater and North Umpqua drainages.

The flows consist of dense, light grey, olivine basalt. The lava welled up from a fissure east of the Pig Iron Mountain — Watson Butte — Elephant Mountain mass. Elbow Butte possibly was the vent from which it issued. The molten lava was extremely fluid and flowed out in copious sheets that rushed away from the vent like thin mud. Some of the material rolled up the drainage of the North Umpqua River almost 3 miles to the base of Kelsay Point, but the bulk of the flows rushed down the valley of the North Umpqua and circled the north boundary of the Pig Iron Mountain — Watson Butte — Elephant Mountain Mass. A portion flowed down the Clearwater River drainage to the south of the mountain mass and rejoined the flows circling the mass to the north. From this point the lava continued down the drainage of the North Umpqua River for several miles.

The edges of the intracanyon flows chilled to jagged, clinkery blocks as the bulk of the lava flowed on. This blocky lava may be seen along the North Umpqua Highway in the vicinity of Lava Creek and Clearwater Falls. At the latter place, the Clearwater River tumbles over the moss-covered, jagged blocks creating one of the most scenic spots in the Umpqua National Forest.

The valleys surrounding the Pig Iron Mountain — Watson Butte — Elephant Mountain Mass, previously eroded by streams, were now partially filled by these lava flows, giving them flat floors. These flows produced the broad level areas such as Thorn Prairie to the north of the mountain mass; and Mowich Park, Toolbox Meadows, and Fish Creek Desert to the south of it.

Toketee Falls cascades over columnar jointing of intracanyon basalts.

The sudden alteration of the configuration of the stream valleys disrupted the drainage pattern in the area. The streams draining the surrounding mountain mass now abruptly disgorged onto the broad valley floor instead of continuing down their old water courses to the bottom of a deep canyon. These streams hugged the contact between the old mountain mass and the fresh lava beds, and flowed westward along this contact. Since the flows filled the North Umpqua and Clearwater valleys, the subsequent stream erosion along Clearwater River and Mowich Creek to the south of the mass, and along Loafer Creek to the north of it, has carved new, narrow canyons as much as several hundred feet deep along these contacts. The waters of the North Umpqua River were diverted along the contact at the northern edge of the flows between the intracanyon basalt and the High Cascade and Western Cascade rocks.

The tributaries of the Clearwater River which drain the area south of Mowich Park and southeast of Fish Creek Desert flow directly across the flat top of the intracanyon basalts and plunge abruptly into the steep canyon of the Clearwater River. Because the erosive power of these streams is less than that of the larger river, they have cut into the rocks much less, and upon reaching the deeper canyon tumble sharply into it. Watson Falls, almost 300 feet high, is formed where Watson Creek drops from its shallow stream channel over the lip of the Clearwater River Canyon.

Fish Creek, which skirts the Fish Creek Desert on the south, flows along the boundary between the intracanyon flows and the older rocks to the south. It appears that the major streams existing at the time the intracanyon flows filled the canyons were diverted along the margins of these flows; and the smaller streams which developed subsequent to the lava flows wander over the surface of the intracanyon lavas.

Columnar jointing on Pig Iron Mountain.

Columnar jointing near Soda Springs.

A very pronounced aspect of the intracanyon flows is the prominent development of columnar jointing. Most of the columns are vertical, and range up to 50 feet in height and 2 to 3 feet in diameter. The columnar jointing is particularly well exposed at Soda Springs, about 5 miles west of Toketee Ranger Station.

This jointing, in large part, is responsible for the development of waterfalls such as Toketee Falls, Slide Creek Falls, and Watson Falls. As the rock weathers the columns tend to collapse, leaving sheer vertical walls. The height of the waterfalls plunging over these walls is dependent upon the height of the columns; the height of the columns, in turn, is dependent upon the thickness of the flow in which they formed. The lofty spires standing erect between the canyon walls along the North Umpqua River between Eagle Rock and Toketee Reservoir were formed in the same manner.