Why Is The Hill Blue?

May 23rd, 2010 by Harrumpher Leave a reply »

The one-word answer to why the Blue Hills are blue is not plastic. It’s riebeckite.

lestWe sweated a little and learned a lot tripping our way up and down the red-dot tail at the reservation. The guide was geologist Les Tyrala. His ceaseless fascination with rocks inspired him to stop suddenly and repeatedly at remarkable specimens, huge and wee.

Fair notice: He’ll do the same thing at Hingham’s World’s End on Saturday, June 19th.

We’ve hiked those trails, including that particular one, many times. We just didn’t know what was there.

silkmothsideBefore we started, actually before Les opened his trunk with samples and maps for the pre-hike lecture, we got a seasonal treat that has nothing to do with geology. Normally the giant silk moths in the area are out for their pre-dying mating flights in early June. One was posing for us on a fence post.

This beauty was the size of my open hand. It was otherworldly in the face. silkmothfaceClick either moth image for a larger view.

To the trail!

This is to recommend heading out with the experts instead of your brilliant, complete recollections from youth or even a guidebook.

We learned some too about rattlesnakes and cooperheads there. The ranger and helper all added to the lore. There aren’t many rattlers, but since the kill-’em-all edict from the commonwealth was withdrawn, they are coming back. They are mostly on the Houghton’s Pond side. They slither about around this time of year as the young males try to find their own turf. The females mate with several males and hold the sperm for years, using it as they need to, although they only give birth (live not eggs) every few years. Black snakes can hardly wait to find and devour the young.

Before we headed up, Les produced a sketch of the hills. It seems a few hundred million years ago, there was maybe a mile thickness of rock and other material covering it. We hike on the leftovers.

We knew there was glacial moving of the boulders and even smaller rocks, as well as volcanic action. Les stopped repeatedly to show how the rocks (bedrock or country rock in his lingo) changed up the hill.

The nasty old glacier simultaneously left striations and overall smoothed the boulders. You can see the direction (mostly southeast) that the glacier moved, taking the lithic baggage with it.

He was most fascinated with the color changes. He showed us where the true blue granite began and how it deepened going up, until it stopped. That showed the layers of sediment. The blue rock contains the relatively uncommon riebeckite.

Les was also great at finding xenoliths. Those are big rocks where a different stone has been pressed, likely by a glacier into the stone itself. Unlike the smooth, small roundness of the composite pieces in pudding stone, these are often sharp edged. We’ve walked over and one many of those for years and had no idea what that were. Largely, we never noticed.

Likewise, another geologic trick was the quartz striping in the granite. I had thought it just formed with the stone, but it isn’t that simple. Striations and cracks left openings many millions of years ago. The commonly occurring silicon dioxide settled into it and under extreme compression became strips of crystallized quartz. Many of these stripes are higher than the larger rock, because they are harder and erode less.

graniteironOur guide is full of such goodies. For example, the granite walls and benches below the observation tower are of stone from the West Quincy quarries. Their iron ties hold them in place because of technique that Old World masons brought to the quarries with them.

Iron by itself would expand perhaps 8% with rust. That would eventually split and destroy even these huge granite slabs. The masons had learned to line the grooves with lead, which does not rust. Then they drove the iron into the space, where it keeps for centuries.

Les is a graybeard and has been in the geology game his whole career. He says he never ceases to be fascinated by what he sees, hypothesizes and proves or disproves. Several of us asked him questions that seemed to pique his interest. When he didn’t know, he said he’d find out.

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3 Responses

  1. Harrumpher says:

    As a further inducement to go on the next rock tour with Les, be aware he recognizes bird calls…seemingly everything around here. He was less confident of the toad noises, but nailed the bird songs and told us how male orioles each develop a unique tune and keep it lifelong.

  2. Maggi Brown says:

    A HUGE thank you to Geologist Les Tyrala for a fantastic program. Les is a wealth of information and he is very generous in sharing his time and energy. The DCR is very grateful to Les for graciously sharing his time and expertise with the general public and the DCR staff. As they say, Les, “You rock!”. Thanks a million!

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