A Brief Discussion of Log Hill Mesa (LHM) Geology

by Jack Rairden

Most of what I know (or think I know) about the geology of Log Hill Mesa and the surrounding region is what I've gleaned from the tutelage and guided tours by my friend and expert geologist, Bob Neel; and, the reading of Dr. Halka Chronic's book: "Roadside Geology of Colorado", Mountain Press Publishing Co., Missoula, MT (1991).  All errors or misconceptions in the following narrative are my own.

LHM is not a well-defined geographical entity - it is the southeastern limit of the Uncompahgre Plateau which extends from south of Grand Junction to just north of the town of Ridgway.  Perhaps the boundary of the Log Hill Mesa Fire Protection District is a reasonably good definition of the LHM.  (See Maps)

        The topography of LHM is mostly delineated by the Dakota Sandstone formation which slopes northeast from Horsefly Peak to the Billy Creek region at which point it goes underground.  The escarpment on the south end of LHM is due to the Ridgway Fault which runs east and west - one is standing on the Dakota Sandstone at the bottom and at the top of the escarpment.  There is about 1200 feet of displacement.  There also is a north striking fault along the eastern edge of LHM.

        The Uncompahgre Plateau including the LHM area is more accurately described as a cuesta.  In Dr. Chronic’s book, cuesta is defined as: a hill with a long gentle slope formed by a resistant cap-rock and a short steep slope on cut edges of underlying weaker rock. 

        Standing on the southeastern-most edge of the Uncompahgre Plateau at Inspiration Point in Log Hill Village, before you to the east, south and west is a beautiful, awe-inspiring view.  You are surrounded by the majestic San Juan Mountains.  To your left is the rugged Cimarron Range featuring Courthouse Peak, Precipice Peak and many others; straight ahead is the incomparable Sneffles Range with 14,150 ft. Mt Sneffles in the background and the town of Ridgway in the foreground; and, to your right is bucolic Pleasant Valley with Dallas Creek flowing through it.  The valley leads up to Dallas Divide and the western-most peaks of the Sneffles Range. (Figs. 1 - 3)

At your feet is Dakota Sandstone.  This was the shoreline of the western edge of the Cretaceous Sea some 100 or so million years ago.  With time, the seashore moved westward as the mountains to the west were further eroded.  After that, this shoreline was buried in many more layers of sedimentary strata which consolidated into this rim rock layer of sandstone. 

This formation was lifted from sea-level to its present elevation over a period of 60 or so million years of mountain building .  This was followed by the gradual 5,000 ft. uplifting of the vast Colorado Plateau 12 or so million years ago.  Finally, the present features were carved by glaciers only 10 to 20 thousands of years ago.

        At Inspiration Point, you’re standing on top of what’s called a normal fault.  The south block has dropped down relative to here - the blocks move relative to one another.  The exact date of the movement is not known, but it’s a fairly recent occurrence during the time when the San Juan Mountains were being built - they are volcanic in origin.  The volcanic activity is recent compared to the Dakota - about 20 or so million years ago.  The layered volcanics that form most of the San Juans were ash flows and mud flows called lahars - the technical name for the mud-type.  All of these mountains around here are like that.  Between the Silverton Caldera and over toward Creede, there are 13 volcanic centers - at one time there were volcanoes blowing all over the place.  The Silverton Caldera comes all the way up to Red Mountain Pass - that was the rim that remained after the volcano exploded or collapsed. 

        The gray-colored mounds near Ridgway are Mancos Shale - normally Mancos sits on top of the Dakota.  That is sea-bottom clay that was deposited on top of the Dakota as the seashore moved westward.

        The Pleistocene Epoch was the last big event that took place here.  It was the most recent ice age when glaciers came down through Pleasant Valley, and down the valley from the Ouray area.  These were alpine glaciers and were not part of the continental glaciations that covered the eastern U. S.  The volcanoes had been here, but were long gone.  There’s a real good chance that the glaciers came after the Ridgway Fault and gouged-out these valleys.

        Much, if not all, of LHM was covered with ice 1,000  ft. or so thick.  As this very large amount of ice melted, torrents of water were carving the drainages on LHM.  The three main drainages on LHM are: Horsefly Creek and Canyon which forms a mesa-like barrier to the west; McKenzie Creek and Canyon and Fisher Creek and Canyon which run nominally west to east at mid-mesa.  The most productive agricultural areas on LHM have been along McKenzie and Fisher Creeks, probably due to the alluvial soil left by the melting glaciers.

McKenzie Butte and Pinon Ridge on LHM are a different geological story.  They are formed by a sill that came in fairly late in the game here.  A sill is an intrusive igneous rock formed by molten lava which is forced between layers of sedimentary rock.  It spreads them apart and then is injected, somewhat akin to squeezing toothpaste from a tube.  The molten layer is referred to as a sill when it spreads horizontally and as a dike when it turns vertical. - it’s just a change in nomenclature. 

There were three different stages of glaciations and they went as far north as the north end of the Ridgway Reservoir; in fact, an arm went north of that.  The Keller-Hills area east of Log Hill Village is located in a glaciated area.  The Dakota Sandstone was either scoured-off or had eroded or is buried; it’s an entirely different type of material as that at Inspiration Point.  It’s all glacial material that was pushed ahead and bull-dozed out by the glacier.  It’s relatively easy to put a septic system in the glaciated material, in contrast to the situation in Log Hill Village where it might be necessary to blast through the tough sandstone rock.

        Another important geological feature on LHM is the horst (a massive block that is higher than the surrounding land) formed between two faults that extend northward from the south escarpment.  Reference (1) below is a recent scientific assessment of this feature. 

         The westernmost fault is traced at Busted Boiler Draw (Fig. 4) which is located about three miles west of County Road #1.  The horst can be viewed from County Road #24 as shown in Figs. 5 and 6.  The Morrison Formation layers that underlie the Dakota can be viewed at the “cuts” along the right side of the road as you drive down the escarpment.

Ref. (1): PRELIMINARY GEOMORPHIC ASSESSMENT OF THE BUSTED BOILER FAULT AND THE LOG HILL MESA FAULT ZONE, SOUTHWESTERN, COLORADO, PIETY, Lucille A. and OSTENAA, Dean A. (2005) - http ://gsa.confex. com/gsal2005RMlfinalprogram/abstract 86033

A note about water wells on LHM: The drilling of wells is hit or miss over all of LHM – some people have success and others don’t.  The best possibility of getting water is in the Morrison Formation below the Dakota - there are porous sands there.  But at the fault, there’s a good chance that the water will not get across the fault.  One may be pumping water that had been accumulating for thousands of years and will not be quickly recharged.

A note about geological time:  It is difficult, at least for me, to wrap my mind around geological time periods expressed in terms of billions and millions of years.  It gave me some perspective to make a rough calculation of the time it has taken to form the displacement along the Ridgway Fault.

Assume that there is one vertical displacement (earthquake) of one-inch in a human life time (75 years).  In a million years, there would be over 1,100  ft. of displacement or roughly the elevation differential of the Log Hill Mesa escarpment.  By the same logic, there has been about a 10 ft. to 20 ft. displacement since the last glaciations here.