Weathering, erosion and reading topographic maps: the surface processes unit for the NY Regents

Surface processes: the weathering-erosion-deposition sequence

The surface processes unit follows one chain of ideas: rock is weathered (broken down in place), the pieces are eroded (transported), and they are finally deposited (dropped) when the carrying agent slows. Getting the vocabulary right is half the unit, and the rest is reading the particle-size graph and topographic maps. This guide ties the dot-point pages together: weathering and soil formation, erosion and the agents of transport, deposition and sediment sorting, streams and the gradient of the land and landscapes and the regions of New York.

Weathering: physical versus chemical

Physical (mechanical) weathering breaks rock without changing its chemistry: frost wedging (freezing water expands in cracks), abrasion, and root action. Chemical weathering changes the minerals: carbonation (carbonic acid dissolving limestone), oxidation (iron rusting), and dissolving of soluble minerals. The rate rises with a warm, wet climate, with more surface area (smaller pieces weather faster), and depends on rock type (quartz resists; calcite dissolves). Soil is weathered rock plus organic matter (humus); most New York soil was transported by glaciers, so it often does not match the local bedrock.

Erosion: reading the agent from the sediment

The single most useful rule: the more rounded a particle, the farther it has been transported (abrasion wears off the edges), and sorted deposits mean water or wind, unsorted deposits mean ice or gravity.

Deposition and the particle-size graph

When an agent loses energy, it deposits its load largest first. The Reference Tables Relationship of Transported Particle Size to Water Velocity graph shows that faster water carries larger particles; read up from the velocity to the curve, then across to the size. Settling order depends on three things: size (large settles first), density (dense settles first), and shape (rounded settles faster than flat). Slowing water therefore sorts sediment, giving horizontal sorting (coarse near the source) and graded bedding (coarse at the bottom of a layer).

Reading topographic maps

Landscapes and the New York regions

Landscapes are classified by elevation, relief and structure: mountains (high, deformed rock, the Adirondacks), plateaus (high, flat-lying rock cut by deep valleys, the Allegheny Plateau and the Catskills), and plains/lowlands (low, flat-lying, the Great Lakes and Hudson lowlands). Development is controlled by climate (water is the main shaping force) and bedrock resistance (resistant rock stands high, weak rock erodes to lowlands). Use the Generalized Landscape Regions and Bedrock Geology maps to match a location to its region, bedrock and age.

Check your knowledge

Attempt these, then check the solutions.

1. State the sequence weathering, erosion, deposition and define each in a few words. (3 marks)
2. Name the chemical weathering process that dissolves limestone and the acid involved. (2 marks)
3. A point at 280 m and a point at 160 m are 4.0 km apart on a map. Find the gradient. (2 marks)
4. State the rule for which way contour lines bend at a stream and what it tells you. (2 marks)
5. How can you tell a glacial deposit from a stream deposit? (2 marks)
6. Classify a high region of flat-lying rock cut by deep valleys, and name a New York example. (2 marks)
7. State the largest particle a stream can carry and how you read it from the velocity graph. (2 marks)