Carved by Glaciers

This summer, Silvy Zhou, a rising senior studying film, animation, and video at Rhode Island School of Design, sought to answer this question. Guided by Dr. Blake Hodgin, a geochronologist and assistant professor at Brown University, she left no rock unturned in the search to reconstruct the timeline of Block Island’s past which may unlock solutions to its future.  

“I feel like Block Island is the Rosetta Stone of glacial geology of this region,” says Hodgin. “From a geological standpoint, [Block Island] showcases the regional history better and provides this comprehensive insight.”

Cliff erosion occurs in three ways: when loose sand washes away, dense and saturated blocks of clay collapse, or large supporting boulders fall out of the cliffs, causing sections of the cliffside to  tumble down into the ocean . With rising sea levels, temperature increases, more frequent storms and precipitation, Block Island’s cliffs are suffering the consequences of climate change. In a recent report,   NOAA   predicts an 85% chance of an above normal hurricane season in the Atlantic this year. Waves crashing upon the rocky shoreline, run-off dragging loose grains, the pelting force of rain and associated groundwater saturation all contribute to the diminishing cliffs. These processes can accelerate erosion rates due to the geologically young and poorly consolidated nature of the sediments. By dating the geological features of Block Island, Silvy and Blake are reconstructing glacial history and mapping which areas are most vulnerable to erosion so that residents can better plan for the future and tourists can partake in conservation. 

“It’s not a good way to live, in constant fear that your house is going to collapse because of the shoreline. So [we’re] able to take away some of that anxiety, just with education,” says Zhou. “If we do research, can we encourage people to buy property that's not on land that's going to erode away and empower people with the knowledge of being able to make decisions for their homes and for the future?”

Approximately 25,000 years ago, a massive ice sheet covered what is now New England. Along the southern edge of the ice sheet, the ice repeatedly advanced and retreated, depositing the sediment that makes up Block Island today. An initial glacial movement eroded the bedrock of the mainland as it advanced southward and also carved Narragansett Bay out of the soft, silty rocks below. The advancing ice sheet deposited the built up material from the mainland into a belt stretching from present day Cape Cod, Mass. to Brooklyn, New York. The second glacial movement consolidated Block Island’s sediments by bulldozing the previous accumulation of sediments and piling them up high into what is called a push moraine. The ice sheet then overrode those bulldozed sediments, advancing southward before beginning a slow steady retreat starting around 20,000 years ago. The rainbow of chaotically layered sediments visible in Block Island’s cliffs were scattered by glacial outwash—gravel, sand, and clay— that eventually formed the base of the lush ecosystem that blankets the island today. The preservation of such diverse sediments is rare beneath the erosive power of an advancing ice sheet. 

Because of this unique history in which rocks from all over the mainland were transported and condensed in one place, Block Island is a geologic wonder to tourists, locals, and scientists alike. Yet, the challenges the island and its residents face today— cliff erosion and sea level rise— can be traced back to the very recent and complicated geologic history that formed the island. 

“It's an ongoing question in my mind as to how we can kind of merge our history that we're seeking to figure out with the most pressing questions that people have: ‘What can you tell us about susceptibility to sea level rise, cliff erosion, and so forth?’” says Hodgin.

A novel method of geologic dating has been instrumental in Silvy’s exploration of the island’s timing of formation: luminescence dating. When sand particles flowed in glacial currents or were carried beneath the glacier, they were eventually buried and hidden from sunlight. Some types of minerals- such as quartz and feldspar- allow the surrounding radiation to ‘cook’ in the darkness, increasing the luminescence of those mineral grains that make up the sand. Luminescence dating involves finding out when the last time quartz and feldspar mineral grains were exposed to light and therefore dates of the layers of sediment. The only way to measure this luminescence is in a darkroom where one hits the mineral with a beam of light to release its stored luminosity.

“If you have some sand and it's exposed to light, and then you bury it, then it will actually have luminescence grow inside the grain, and how that happens is through radiation,” says Hodgin.  “Radiation basically induces the growth of some luminescence in individual sand grains up to a certain point. That usually takes place over about 500,000 to about a million years, so any sand grain that's less than a million years old, you can generally determine how long it's been buried, or when the last time it was that it saw light.”

Silvy, having just completed a semester of experimental film at RISD, is familiar with the process of developing film in a darkroom.  Utah State University’s Luminescence Lab  has open doors for undergraduate researchers, so Silvy flew to the darkroom in Utah in July to utilize her artistic experience, dating the samples they’ve collected at Block Island. With the help of lab technicians, she was able to purify the samples into feldspar and quartz and prepare them for testing. 

Results of the luminescence dating will come long after SURF ends, as processing can take up to four months.

“Film is photosensitive. If you expose it to light, then it would just get totally washed out, it would destroy the image," says Hodgin. "And the same thing here, if you exposed [the sand] to light, then it would just reset the age of all of those grains of sand."

Besides luminescence dating, Silvy has been illustrating Block Island’s unique geological features during field days. Most detailed maps of the Block Island glacial sediments are outdated or insufficiently detailed, so observational artistic rendering helps map out differences in types of sediments to decide where to sample. It's also the basis for correlating sedimentary units that are shared between Block Island and other neighboring islands. Part of Silvy’s research involved sampling those correlated units on Long Island, Martha’s Vineyard, and Cuttyhunk Island in Massachusetts to test this hypothesis using luminescence dating. In the future, these visualizations of Block Island could take the form of educational material that can show locals and tourists why the island faces issues of cliff erosion. Expanding to a larger scale, those images can also explain how climate change has contributed to the island’s changing scenery. 

“I feel like a lot of people that we've met are kind of skeptical of why we're walking around the beach with a shovel and hammer, people that are like, ‘If you touch this cliff, my house will fall down’,” says Silvy.

By laying the groundwork through dating Block Island’s sediments, Silvy and Hodgin are contributing to a larger knowledge that connects deep geological history to the challenges that Block Island will face. The combination of sea level rise and loose sediments is contributing to accelerated erosion, and dating the sediments is a stepping-stone in helping Block Island residents plan for their future and protect their homes. Block Island locals and tourists cannot just go to the beach, but have an intimate connection with the geological features in front of them.

“This concept of ‘the world is so much bigger than I am, and there's so much out there’ I think it's a really powerful thing for a community to have,” says Silvy.