Tuesday, March 19, 2013

Lake Powell Houseboat Trip - Part 3

The final installment of my house boating trip on Lake Powell features spectacular geology with weathering pits and snow on the lake!

Near Cookie Jar Butte we hiked up on a ridge of Jurassic Entrada Sandstone. The day started overcast but the sun was soon here.

Variably colored cross-beds points the way to Dominguez Butte (with the reflection on the lake surface) and Tower Butte (right).

Beautiful cross-beds in the Entrada Sandstone. Note the differential weathering on this outcrop. Some horizons stick out a bit farther than others meaning that they are more well-cemented. These tend to be the lighter colored bands. The more orange bands are softer and less-cemented, and so weather faster than some other cross-beds. But not only that - the differences in cementation can be attributed to the specific way in which each horizon accumulated. The orange, less-cemented beds were formed as ripples on the leeward side of the dunes. The lighter, well-cemented beds were formed when the crest of a dune avalanched down-slope. These avalanche beds have grains of various sizes and so transmitted groundwater better than their windblown cousins. Thus, they obtained more cement from groundwater.
 
Patterns in the weathering pits formed along cross-bedding in the sandstone

Barb and Helen on the rocks

These weathering pits have likely formed where sandstone pipes are located in the Entrada Sandstone. The sandy material that once filled the pipes was less cemented than the surrounding sandstone and so the pits are formed when exposed.

Trip portrait (minus Darryl and Wayne)

Helen, Barb, and Art for scale

Ominous weather

Until the sun sneaks out

What a place this is. As it turns out, it began to rain at about 5 PM on our last evening on the lake and it rained pretty good. As we went to bed, we could hear the rain drops on the houseboat but then about midnight, the rain stopped.

It stopped because the rain turned to snow! We kept listening to the weather band and they said snow level at 4,500 ft., well above our elevation. But it got colder than anticipated and on our last morning we woke up to a surprising look for the lake.

At first the rocks were hidden by thick clouds

But as the morning wore on, the buttes began to reveal their winter splendor

Wow! It was a land transformed from the bright Kodachrome colors we had seen the first 6 days of the trip

Reflections with snow on Dominguez Butte

Barb and Helen in  farewell pose. Winter is losing its grip on us here in Arizona and on March 20th we will welcome the spring equinox! Life renews and refreshes from winters icy grip. Thanks to Gary Ladd for leading this trip and if you would like to spend a week with the Master of the Lake, please contact the Glen Canyon Natural History Association here. Gary Ladd is an instructor and has trips with openings scheduled this year!

3 comments:

z.schierl said...

Excellent pics! Any idea what causes the bright white interbeds in the sandstone? Do they also weather faster than the orange bands? I can't quite tell in the photos...

Wayne Ranney said...

Dear Z: The white interbeds seem to weather at the same rate as the orange ones but the grain size is likely different in each causing the stripped pattern. WR

Um andorinhão não faz le printemps? olha que nã óia ki nã... said...

The white interbeds seem to weather at the same rate, but the orange ones should be cemented strongly no?
the red color indicates iron oxides ergo....