Boston Harbor’s Rocky Islands

September 22, 2010


Photographer: Rob Sheridan
Summary Author: Rob Sheridan

Scattered among the nearly three dozen glacial till islands and terrestrial drumlins in Boston Harbor Islands National Park (Massachusetts) are a few hard rock outcrops and rocky navigation hazards.The Park contains both terrestrial and undersea portions of a massive drumlin field that occupies much of the Boston Basin. The drumlin field was created by two retreating ice sheets; one about 150,000 years ago and the most recent retreat about 15,000 years ago. Most of the Park’s islands are composed of glacial till from these two episodes of glaciation. The scattered hard rock structures have a completely different, much older, origin. Although exact dates are unknown, the approximately 600 million-year-old layered mudstone argillite that forms much of Boston basin's bedrock was intruded by igneous dikes and sills. This occurred perhaps 400 million years ago. Subsequent compression and erosion distorted and exposed portions of these durable dikes and sills. Such events explain the frequent juxtaposition of distorted mudstone argillite and hard basaltic rock in multiple islands and outcrops throughout Boston Harbor. One such complex igneous structure is pictured here, in a view of Boston’s Outer Harbor looking southwest from Green Island. This is a small rocky island used mostly by breeding cormorants, gulls, and eiders and the occasional resting harbor seal. In the background, across the Hypocrite Channel, is the similarly rocky Little Calf Island. In the far distance, the more typical glacial drumlin islands of the Boston Harbor Islands National Park can be seen. Photo taken on September 16, 2010

Photo Details: Camera Maker: Canon; Camera Model: Canon PowerShot SD1000; Focal Length: 5.8mm; Aperture: f/2.8; Exposure Time: 0.0031 s (1/320); ISO equiv: 80.