Sandspits and tombolos are coastal landforms created by the deposition of sediment, primarily sand, by longshore drift. Both are dynamic features that reflect the interplay of waves, currents, and sediment supply.

Sandspits: A sandspit is a narrow, elongated ridge of sand or shingle that extends from the mainland into a body of water, typically across the mouth of a bay or estuary. They are formed by:

1. Longshore Drift: The primary mechanism is longshore drift, where waves approach the coast at an angle, pushing sediment along the shoreline. As the waves retreat, they carry sediment back down the beach, but the net movement of sediment is along the coast.
2. Change in Coastline Orientation: When the coastline changes direction abruptly, such as at a headland or the entrance to a bay, the longshore current loses energy and can no longer carry its sediment load effectively. This causes the sediment to be deposited.
3. Deposition: The deposited sediment gradually builds up, extending outwards into the water. The spit often curves inwards towards the land due to the influence of waves and currents within the sheltered water of the bay or estuary, forming a 'hooked' or 'recurved' spit.

Spits can grow over time, sometimes partially or completely enclosing a bay to form a lagoon or salt marsh. Examples include Spurn Head in the UK.

Tombolos: A tombolo is a depositional landform that connects an island or a stack to the mainland or to another island. They are also formed by longshore drift and wave action:

1. Island Obstruction: An offshore island acts as an obstruction to incoming waves. As waves approach the island, they are refracted (bent) around it.
2. Wave Refraction and Sediment Deposition: The refracted waves meet on the sheltered side of the island, where their energy is significantly reduced. This reduction in wave energy causes the longshore currents to slow down and deposit their sediment load in the calmer water behind the island.
3. Building a Bridge: Over time, this deposition builds up a ridge of sand or shingle that gradually extends from the mainland (or another island) towards the offshore island, eventually connecting the two. If the island is connected to the mainland, it's a simple tombolo. If it connects two islands, it's a double tombolo.

Tombolos can be single or multiple, depending on the number of islands and the direction of wave approach. A famous example is Chesil Beach in Dorset, UK, which connects the Isle of Portland to the mainland.