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-18- method was carried out in the early phases of the work and the degree of accuracy further justified its use. While recognizing that the types of surficial units are neither homo- geneous nor discrete with respect to texture, three principal types were focused on here. (1) Ponds and swamp deposits generally occur as isolated units or along reaches of streams. Rainfall on these areas rapidly becomes part of streamflow or is otherwise detained at the surface owing to low permeability of the substrate. (2) Impervious deposits include bedrock, glacial till and lake beds, and some fine-grained, postglacial marine deposits. Bedrock and till usually occur in upland areas, while lake beds and marine deposits are found in lower areas associated with swamp deposits. Runoff to streams is relatively rapid from these materials. (3) Pervious deposits include glaciofluvial sand and gravel, and are found adjacent to upland areas as kame terraces, along streams as valley train and modern stream terrace deposits, and as scattered positive relief elements in lowland areas. Runoff to streams is relatively slow because of depression storage on irregular surfaces and high infiltration capability of the coarse-grained materials. The abundance and distribution of these types of materials in a watershed can be expected to have a profound influence on the ultimate amount of runoff resulting from rainfall, snowmelt or a combination of the two. The areal extent of these surficial units in the Neponset basin is illustrated in Figure 5. The amount of pervious deposits in this water- shed is about average for the study region, although considerable varia- tion exists among the 18 basins. The typical abundance of well-sorted, . .