EVALUATING GEOMORPHIC CHARACTERISTICS OF DIANA'S POOL, A BEDROCK-CONTROLLED RIVER IN CHAPLIN, CT

The geologic characteristics of many river landscapes in Connecticut are strongly influenced by the underlying bedrock. This study examines a bedrock controlled reach of the Natchaug River in South Chaplin that is within the Diana’s Pool Nature Preserve. Here, differential weathering is evident, leaving behind overhangs, which eventually break and leave slabs along the river bank. The influence of the bedrock on the river is reflected in the shape of the long profile, the character of valley-side breakdown, and the influence the mineralogy of the bedrock has on erosional forms made by the river. A variety of field and analytical techniques were used to evaluate these controls.

The long profile of the river was surveyed with a laser-total station in order to identify deeper “pools” and steeper “steps,” as well as providing location control for other measurements. A distinct break in slope occurs when the river enters a confined, fault-controlled bedrock valley. A total of 132 boulders were examined along the east bank as their shapes reflect the process of weathering, bedrock breakdown, and transport by the river. Shape analyses reveal disk as being the dominate boulder shape, accounting for two-thirds of the measured boulders, and blade comprising the other third. Sphere and roller shape rocks where infrequent with only 6 boulders. As well, there appears to be no trend among the shape of the boulder and the shift onto different bedrock units. Spreadsheet analysis of boulder dimensions also reveal that the boulders, if derived from adjacent bedrock valley walls, could not have broken by beam failure because their shapes do not fall within the limits required for failure based on common tensile strength values for these rocks. This implies that valley-side breakdown was greatly enhanced by the presence of joints and fractures within bedrock. Erosional forms are best preserved in resistant feldspar-rich lithologies and are most numerous near the current river. However, some large and weathered forms do occur at higher locations suggesting that they may have formed by meltwater during deglaciation. Microscope thin-section analyses indicate that differential weathering patterns and bedrock control on the valley relate to differing mica or feldspar compositions.