Erosion and Deposition

Weathering makes the loose pieces. Erosion is what happens next, when an agent picks up the weathered material and carries it somewhere else. Deposition is the end of the trip, when the agent runs out of energy and lets the sediment settle.

The whole thing runs on energy. The more energy an agent has, the more sediment it can carry and the bigger the pieces it can move. A raging flood rolls boulders. The same stream at a trickle can barely move sand. When the agent slows down and loses energy, it drops what it is carrying, and the biggest, heaviest pieces drop first.

Five agents move sediment across Earth's surface. Two of them leave a clue you can read later in the deposit itself, which is whether the sediment is sorted or unsorted. That is one of the most useful clues in this whole unit.

Running water is by far the most important agent, and the one that has shaped most of New York. Streams and rivers erode their channels and carry sediment downhill toward the sea. Water sorts its load by size as it goes, so water deposits are layered and sorted.

Glaciers are moving ice. They are powerful erosional agents, and because ice carries everything from clay to boulders frozen together, glacial deposits are unsorted, a jumble of every size mixed together.

Wind carries only fine material, mostly sand and dust, and sorts it well, building dunes. Waves shape shorelines, moving sand along beaches. Gravity pulls material straight downhill in landslides and slumps, a process called mass movement, and it powers every other agent by pulling water and ice downslope.

For running water, one thing controls its energy more than anything else. That is velocity, which means how fast the water moves. Faster water carries more sediment and larger particles. This relationship is one of the most tested ideas in the course. The older Earth Science tables graphed it directly. The 2026 Reference Tables do not, so learn the pattern itself.

The rule is simple and worth memorizing. As stream velocity increases, the largest particle the stream can carry increases. A slow stream carries only clay and silt. Speed it up and it can move sand, then pebbles, then cobbles. A fast flood can roll boulders. The graph below shows that climb.

Slide the stream velocity up and down and watch which sediments the stream can carry. Anything too big for the current is left behind, which is deposition. This lab follows that same relationship.

When a stream slows down, it deposits its load, and the order it drops things in is not random. Three properties decide what settles first:

Size. Big particles settle first, small ones last. Density. Denser particles settle faster than lighter ones of the same size. Shape. Rounded, spherical particles settle faster than flat or angular ones.

The biggest and densest pieces drop first as the water loses energy. So a single flood often leaves a layer that goes from coarse at the bottom to fine at the top. This is called graded bedding, and it is a clear sign of water deposition. The same sorting happens along the length of a river. Coarse gravel drops in the fast upper river, and only fine mud makes it out to the calm water at the mouth.

You cannot understand New York's landscape without glaciers. During the last ice age, a continental ice sheet more than a kilometer thick covered the entire state. It did not fully retreat until about twelve thousand years ago, and it reshaped the whole map.

Glaciers erode in two ways. First, they pull rock loose and freeze it into the ice. Second, they use that trapped rock to grind and scratch the bedrock underneath. Those parallel scratches are called striations, and they point in the direction the ice moved. Where a glacier flows down a stream valley, it carves a wide, flat-bottomed U-shaped valley. That is deeper and straighter than the V-shaped valley a river cuts.

Glaciers deposit unsorted material called till, dropped straight from the melting ice with every size mixed together. Ridges of till dumped at the edge of the ice are moraines. Long Island is largely a pair of terminal moraines, the piles of debris marking the farthest reach of the ice. The Finger Lakes are old river valleys that glaciers gouged deep and then dammed with till. Boulders carried far from their source rock and left stranded are called erratics, and they dot fields across the state.

Wind is a weak agent and can lift only fine sand and dust. But it sorts what it carries very well, and it piles that sediment into dunes that migrate downwind. Because wind moves only small particles, a well-sorted deposit of fine, frosted, rounded sand is a clue that wind did the work.

Along coasts and lake shores, waves are the main agent. They grind rock into sand, carry it along the shore, and build beaches, sandbars, and spits. New York's ocean and Great Lakes shorelines are shaped and reshaped by this constant wave work.

Erosion carries weathered material away. Deposition drops it when the agent loses energy. Running water is the main agent, and its power depends on velocity, so faster water carries bigger particles. When water slows down it deposits, and it drops the biggest, densest, roundest particles first. That sorts and layers the sediment. Glaciers do the opposite. They drop everything at once, leaving unsorted till, and they carved the New York landscape we live on, from the Finger Lakes to the moraines of Long Island. Sorted and layered sediment points to water or wind. A jumble of all sizes points to ice.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.